Yeah, RWD cars understeer into a corner when speeding, as was intended when they are manufactured for safety reasons, but if you're in a RWD car with and you mash the throttle in the middle of a corner, what's it honestly going to do? Understeer more? Brake? Or maybe create a rainbow and automatically drive to the moon, where it has tea with deceased racing drivers?
It'll oversteer, it's simply physics. When regular people experience understeer or oversteer, they will brake like hell, it's natural.
Cars are manufactured to understeer because (amongst other factors) because of that. I'd rather be sitting next to someone braking under understeer, than braking under oversteer.
Conclusion:
You're taking the wrong arguments into the wrong context Jeff. Any decently built car will understeer naturally for safety reasons, but as was said: if you intentionally want to make a car oversteer, it won't be too difficult in a powerful RWD car. It's practically impossible to do in GT4 though, unless you use a crapload of momentum to swing the ass of the car around, don't you agree?