Wheels and exhaust are acceptable mods I believe even to purists. So long as they are sensible (ie. no 'twin exit' exhaust systems, no cheesy chrome wheels etc.) then why not?
Fair enough, the OP wanted to hear about totally stock cars, but how many people on here are really bothered about a Supra having aftermarket wheels and exhaust?
Whenever you read about Supras in 'mainstream' motoring journalism (EVO for example), the one thing thats always mentioned is the huge tuning potential. As with any iconic car its always nice to have original examples, but sensible, tasteful mods never do any harm IMHO.
Forgetting all that, I wonder how many ultra-rich collectors (Jay Leno springs to mind) have mint original Supras in a bubble somewhere? Toyota presumably keep afew somewhere too.