What are those tremendous differences that you speak? the last time I checked both Porsches were very docile, no big differences in the drivetrain except for the expected performance, even both cars were sharing the absence of lift off oversteer at cornering, a signal of identity of those cars, specially pronounced on old models like the 993 gt2..
The differences I'm speaking of are, for example, that the 997 GT2 felt way more prone to understeer when entering corners, much like you'd expect from a modern road car, where safety is a much bigger concern for the initial setup. Also, the general tendency to oversteer is much more pronounced with the 993 GT2 - overall, it felt more lose, so to speak. With the default setup and no upgrades, of course.
I just want to rebutt the claim that cars in Forza feel the same when they share the drivetrain layout, because, franky, that's not true, at least not to my experience. You can also clearly feeln the difference between the more understeery and stablle Audi front engine - AWD drivetrain and the more lose but also more neutral front engine - AWD drivetrain of an Impreza. Again ,at least according to my experience with the game.
Whether the difference is as pronounced in Forza as it is in GT5 is something I don't really want to get into now, as that would also raise the question how different the cars feel in real life and how much of a difference should be considered realistic (which could probably again lie somewhere inbetween both FM3 and GT5).
But acting like the cars in FM3 fall into one of the six drivetrain catagories (FF, FR, MR, AWD, M-AWD, R-AWD) and all cars in a given catagory feel the same, that's just plain wrong and doesn't match with my experience with the game at all...