First some disclaimers; I am not a professional competitive driver, I have had no formal racing driving instructions, my GTS DR/SR is merely C/S, I race very infrequently on GTS, I will not address drifting.
With that, I attempt to answer your primary questions.
Q1 - I did not switch from automatic to manual gear changing when beginning GT. As IRL driving, I initially learned using a manual transmission. I have been of driving age for more than ⅔ life, so shifting gears is very familiar to me.
Q2 - Button control assignment largely is up to you. I have used a wheel since GT6. While still using a DS3 controller, I shifted up using R2 & down with L2. This method seemed to closely replicate the motion using wheel mounted paddle shifters.
I controlled steering with left stick. I accelerated using right stick up & braked with right stick down. Selection of other functions seems inconsequential here.
Gear selection & most other actions are discreet processes meaning either that they move through stepped distinct increments or that they exist exclusively in on/off states. My button assignment choice for these actions was buttons whose resolution of & magnitude of range is very small. I reserved the analog sticks for speed control inputs since I desired the maximum variety of input amount & control. I hope you can see the sense of this.
Q3 - I am not sure exactly what you mean by asking how much attention I require to play. I assume you wonder how greatly I concentrate on my gear selection while driving. Based on my current skill level & experience level, I will assess my attention level, while driving, to be an average of 3 on a 10 scale. It is important but, with practice leading to track familiarity, not overwhelming. I well anticipate what to expect from my inputs, so I more adeptly can react to situations on track without having to wonder what gear I need.
Q4 - I really cannot accurately assess how much time will be required for you to develop an effective gear selection strategy. You’re A rating combined with the long duration you have played should make you a quick study. Instead, I will discuss some aspects of driving & of racing which impact my manual selection of gears.
Assuming your interest remains high concerning a switch to manual gear selection, I also urge you to refine your choice of gear ratios. This underlies your selection of gears & must be discussed first. To work the best with what you have, have the best you can.
You should install the fully customizable gearbox. The ratios you choose largely depend on two factors, your car & the track. The greatest determining factors about your car are its power & its torque. Generally speaking, more power enables greater speed & more forceful acceleration. More torque will enable you to use effectively a greater portion of each gear, specifically lower rpm range. Remember that speed of car A entering turn X will be different than that of car B getting into same turn. I really suggest you practice using lower power cars first, then moving to Gr1 monsters. I think no single gear set will apply universally to all cars & tracks.
To the tuning limits the game allows, you need to utilize most of the top gear of your car upon reaching the fastest portion of the track. Remember that the ratios of both individual gears & the final drive may be adjusted. The tuning area display visually should make clear the effects of these changes as they relate to one another. Experimentation will help you understand the effects as they relate to your car's performance on the track. Set the lower range of the lowest gear possible to the slowest point on track. Typically, the game will not allow you to stack the gears so closely to use all of them. Do your best to adapt.
Your selection of the middle gears is more complex; the track's individual sections should dictate most of this. During corner exit, you often desire to input sudden full throttle acceleration. Low rpm sacrifices some power, but largely prevents undesirable wheel spin. Try to determine what you expect your exit speed to be & align the lower end of a gear for it. Some drivers opt to enter the corner one gear lower than they intend to exit, shifting up immediately upon exit. I have employed this method occasionally when I get in too hot, but I prefer to enter a corner in the gear I need when leaving. Some longer carousel turns will allow shifting in the through section but be cautiously choosy shifting while cornering. The power curve mentioned earlier will limit how low rpm you can pull out of a turn. Sometimes you will exhaust your supply of gears to optimize before you run out of distinct corners. Again, do your best to adapt. Don’t assume that your gears must be exactly evenly spaced apart from one another because not every track is High Speed Ring. The Nordschleife will challenge your utilization of a customized gearbox.
For corner entry, choose ratios so that you are toward the top end of a gear. The engine compression braking during the initial rpm reduction will aid your brakes in slowing down your car. These points will serve to advance to examine shifting strategy.
Listen to your engine when in different ranges of rpm to become familiar with how your car engine sounds so that you can estimate rpm based on sound alone. This allows you to focus your eyes on the road, not your display. (game options allow you to customize engine sound level & that of some others) You need your behavior driving while using manual gear selection to become automatic
, that is, to become so natural that it simply is second nature. Typically, your best option during extended acceleration is to run a gear near its top limit before shifting to the next gear. Refrain from downshifting before the next lower gear will be in a workable range; loss of traction can occur entering a gear in the maximum upper range. Sometimes for corner combinations, the exit of one corner becomes the entry to the next. A higher rpm between allows a throttle lift to reduce speed more gently & more effectively (so as not to upset the cars balance) than braking for a following corner.
Keep in mind that some engines will overrev well while some others will bump the rev limiter & rob you of power. Entering a corner, even though you might be revving past the maximum power of the gear, less preferable is upshifting for 800 milliseconds before you need to brake hard. Also, upon an event where you must abort your attempt at expected continued acceleration (possibly even having to slow down slightly, traffic might hold you up), sometimes holding a gear helps. When clear, your car might accelerate better than if you have shifted & must resume acceleration from low range of a higher gear.
Sometimes short shifting (upshifting before max rpm) helps. Combinations of bad tires, high (especially very peaky increase) power during increased rpm, low surface traction sometime make any trip to high-rpm-ville sketchy, especially in lower gears.
Q5 - I suspect your racing ability both in terms of qualie lap time & in terms of negotiating time-increasing impediments will be enhanced. Situations which slow your pace could be traffic, worn or low-quality tires, decreased road surface traction conditions like rain or having driven wheel(s) off tarmac during corner exit overrun or a desire to conserve condition of tires &/or of fuel.
In short, ***make the change***. Don’t continue to let the car decide what's best for your situation when you know better. You will wonder why you never fully committed to learning this earlier.
I have not read all previous posts before posting this. I know several already have made some of these points. Others might disagree with my long winded advice.
Many of us are not professionals. I'd like to hear the thoughts of others.
Thx for reading