Basically I do not agree with your weight distribution theory.
I say (and this is based on IRL experience):
Nose-heavy = understeer (allmost every Audi)
Tail-heavy = oversteer (Ruf Yellowbird, an E30 Touring)
👍 Yep, not just based on IRL experience, but also theoretic as putting in a lighter engine will reduce understeer dramatically, as for Audi, they used to put the engine far up in the front of the engine bay ( even slightly in front of the front-axle ) and only very recently put the engine further back for better weight distribution.
Makes me wonder if this engine-swap feature ( which I have to say gets me more excited when I think of the possibilties, given that it will be accurately and realistically implemented to PD's usual standard ) if indeed implemented also allowes a certain amount of freedom where to place the engine/gearbox.
I wouldn't mind converting an AWD car into a RWD, but not the other way around ( similar for FF or MR, RR ) as there probably should be some limits applied but I wouldn't mind putting in a bigger engine and decide to place it ( within the physical limits of that car ofcourse ) slightly further back ( or more to the front ) and fit the ( custom ) gearbox closer to the rear-axle to fundamentally change the weight distribution in the way you see fit.
This also happens for example when converting a road car into a touring car racer ( the only similarity often being the styling of those cars ) and for those doubting the realism of engine swaps/customization, just do some research and you'll find it is very common.
Not just DIY ofcourse ( as there are limits in knowhow, technical resources and finance ) but also professional racing/tuning companies do it all the time.
I think this is what makes a fundamental difference in what people think is real or desired regarding this feature in GT5, whether they see themselves, whilst playing, as themselves with more resources perhaps than IRL but still having a DIY approach to car tuning/customization ( nothing wrong with that, btw ).
Or they can imagine, whilst playing, being a professional cartuner with all sorts of possibilties they couldn't dream of IRL ( but still feasible/realistic ) made quite easy within the context of a game.