Could you be specific to which of the races you are forced to drive a race-specd stock car in order to be competitive?
From the GTPlanet Game Guide:
Gran Turismo All Stars
Observed Opponents
* Mazda 787B Race Car '91
* Nissan R89C Race Car '89
* Mazda RX-7 LM Race Car '01
* BMW V12 LMR Race Car '99
* Nissan R92CP Race Car '92
* Minolta Toyota 88C-V Race Car '89
Dream Car Championship
Observed Opponents
* Mitsubishi FTO Super Touring Car '97
* Pagani Zonda LM Race Car '01
* Toyota RSC Rally Raid Car '02
* Honda NSX-R Prototype LM Race Car '02
* Subaru IMPREZA Super Touring Car '01
* Mazda RX-7 LM Race Car '01
Like The Wind
Observed Opponents:
* AMG Mercedes CLK-GTR Race Car '98 - Does not pit
* Audi R8 Race Car '01 - Pits lap 4
* Bentley Speed 8 Race CAr '03 - Pits lap 3
* Ford GT LM Race Car '02 - Does not pit
* Gillet Vertigo Race Car '04 - Does not pit
* Jaguar XJR-9 Race Car '88 - Does not pit
* Mazda 787B Race Car '91 - Pits lap 3
* MINOLTA Toyota 88C-V Race Car '89 - Pits lap 3
* Nissan R390 GT1 LM Race Car '98 - Pits lap 4
* Nissan R89C Race Car '89 - Pits lap 3
* Nissan R92CP Race Car '92 - Pits lap 3
* Playstation Pescarolo C60/Judd Race Car '04 - Does not pit
* Pescarolo Courage C60/Peugeot Race Car '03 - Does not pit
* Peugeot 905 Race Car - Does not pit
* Sauber Mercedes C9 Race Car '89 - Pits lap 3
* Toyota TS020 GT-One Race Car '99 - Pits lap 3
Formula GT World Championship
Observed Opponents:
Formula Gran Turismo '04 (different colours)
Real Circuit Tours
Observed Opponents:
* AMG Mercedes CLK-GTR Race Car '98
* Audi R8 Race Car '01
* BMW V12 LMR Race Car '99
* Ford GT LM Race Car Spec II '04
* Mazda 787B Race Car '91
* Nissan R390 GT1 Race Car '98
* Nissan R89C Race Car '89
* Peugeot 905 Race Car '92
* PlayStation Pescarolo C60/Judd Race Car '04
Gran Turismo World Championship
Observed Opponents:
* AMG Mercedes-Benz CLK-GTR Race Car '98
* Audi R8 Race Car '01
* Chaparral 2D Race Car '67
* Chaparral 2J Race Car '70
* Ford GT LM Race Car '02
* Gillet Vertigo Race Car '04
* Mazda 787B Race Car '91
* MINOLTA Toyota 88C-V Race Car '89
* Nissan R390 GT1 Race Car '98
* Nissan R89C Race Car '89
* Nissan R92CP Race Car '92
* Pescarolo Courage C60 Race Car '03
* PlayStation Pescarolo C60 Race Car '04
* Sauber Mercedes C9 Race Car '89
* Toyota GT-ONE Race Car (TS020) '98
You get the idea, I could post the others but I don't want this post to go longer than it already has. Look the the
game guides to see what I am talking about. I really never understood why more races couldn't be like this:
Clubman Cup
Observed Opponents:
* BMW 330i '05
* TVR V8S '91
* Mercedes-Benz AMG 190E 2.5 16V Evolution II (DTM) '91
* Alfa Romeo GT 3.2 V6 24V '04
* Dodge Neon SRT-4
* Clio Renault Sport V6 Phase 2 '03
* Nissan SKYLINE GTS-t Type M '91
* Fiat Coupe Turbo Plus '00
Normal-ish cars racing around the track, cars real people might actually own and race. I don't know about you but I do not have an Audi Le Mans car sitting in my garage...all I got it a Mini Cooper.
You are very much making it sound like that is the normal situation, and that simply is not the case... far from it. In fact, if you have issues with being forced to use particular cars and set-ups, then if anything GT4 should be your favorite driving game as it more than any other racing game offers the absolutely widest variety of choices in terms of the pool of AI cars you are up against, the selection of cars you can use, the tuning choices you can make, and the mods you can use.. or not use. Really, you observations so far suggest you may have either forgotten all the aspects to GT4, or may not have played it much, or some other reason, as there is ample evidence to prove what you claim is not true.
This shows your bias for the game, if you want to win many of the races in the Extreme Hall you more or less have to use a race spec car. You can use a normal car but you either have to add so much power to it that it is impossible to control or you just can not beat the other cars.
I've shown you several races in which a race spec car is required to win the race, like I said I can show you more but I feel like you should get the idea. It's not me being biased against the game either, I hate the higher level races in Forza for exactly the same reason.
There is certainly nothing wrong with preferring one game over another, but please use actual facts if you want to support that opinion.
The only reason I prefer Forza over GT is because I feel it's more fun, but the game is far from perfect when it comes down to physics.
Quite true, and of course there are sooo many aspects need to create a real driving simulation that no console will ever be able to do. Even $100,000 pneumatic simulators still can't completely recreate all the aspects of actually driving.
Then GT should not call itself "The real driving simulator" because it is not.
So here appears to be the big problem. Your making a massive leap of faith believing that all cars in general are going to behave roughly the same way, and that's as far from the truth as you'll ever get. In brakes alone, there is a huge difference between different types, as there are with tires, transmissions, suspensions, chassis, etc, etc etc.
The 20 or so cars I've driven in real life that are in the game all point to the same thing. They were all stock as well since a majority of the people I know do not modify their cars or I was driving one off the dealers lot. If 20 cars say the same thing about virtual physics then I'm inclined to believe the real life car is right and the game is wrong (both Forza and GT).
By the standard you suggest, anyone who has ever driven a car can equally determine how realistic a driving simulator is... even though it has different cars, different set-ups, different racing conditions, and different tracks.... which of course is wrong on so many levels, and ignores the hundreds of thousands real life variables.
So you are saying just because you drive a real car you have no idea what it feels like until you drive it on a track? That is quite possibly one of the dumbest points I've heard thus far, if you own a car you know how it feels. I've driven the Mini Cooper in the game and it feels nothing like the real thing, this goes for both Forza and GT as well. And I'm not talking about adjusting the cars in the game, I'm talking about stock car in the game v. a stock car in real life, set up how you would drive it off the show room floor.
So clearly at least at this point, your basis of what is realistic has very little to do with what the games have, and thus your comparisons to real life are extremely limited, and thus, understandably are far more reflective of your personal preference and not compared to having actually driven the same cars on the same tracks... which would be the only reasonably accurate way of comparing how realistic a game is.
Once again because I have not driven the cars around a track that makes you right and me wrong?

I guess I'll just continue driving my car even though I have no idea how on earth it is supposed to feel in real life.
I really am not trying to put your opinion down and certainly not trying to have an argument, only that is important to distinguish between a personal opinion and preference to actual facts and making accurate comparisons.
You really could have fooled me.