In GT concept (which I play the most quick and easy fun) with all the cars that are RWD or 4WD if you use the TCS and ASM you will have this :
1) a lot less fun, no powersliding,no drifting
2) slower lap times even if the vehicles are easier to handle what those things do is actually braking all the time without you knowing it
ASM is really a beginner driving help and it should be turned off as soon as you start to win races.
It's like a real-world car ESP it does not help for a sporty driving it slows you down a lot that's it, as in GT safety is not an issue I strongly suggest you disable it.
TCS can be used but only when you can set it to a lower setting than standard otherwise it will also slow you when you start re-accelerating.
One day I tought "hey why not try to play the game without TCS and ASM since I've got a logitech wheel I should be able to handle spin-offs better" and wow..in 3 laps on autumn ring with the skyline R34 not even trying to make a best lap ,only having some fun not only did I gain 7sec per lap on a TCS-ASM record it took me hours to do
but I end up with a nice replay where I'm drifiting most of the corners, mainly because I drove like a pig lol
It took me all three Gran Turismo's to seriousely try driving with no ASM stuff but believe me the game is a lot more fun now..bye bye understeer hello powerslides !!!
Ireally don't know why most of my friends simply refuse to try plaiyng without it when I watch my RWD replay and theirs I feel really sad for them..and my driving is really not that good.
edit:
I love Zonda cars without ASM & TCS they are really fun to play stock no need to tweak them or stuff choose them and drift !!!
I don't know about the racing cars tough I mostly play with streetlegal cars
