Tuning is way over rated in GT5P. Usually people will run slower times than those set in arcade mode TT, where tuning is not available, with comparable pp.
More often than not, I race online with different tires, weight, and power just to coincide with the PP setting for that race, and everything else nearly stock. I will change gearing some to fit the track, just in case I can save a few shifts, or I will add .05 toe, or .5 camber, or change the suspension settings by 1 click, and thats it!
When I see guys post their tunes with front springs set at 10, I think "no wonder you are getting smoked every race".
I'll take the NSX for an example. It is 547pp all stock on S2 tires. TT times are in the low to mid 15's, I think. But at the 550pp HSR race a month or 2 ago, you would be hard pressed to see an NSX running in the mid 15's without the draft.