For a FR with M3's size and height, the L6 at the front is much more lighter and should make it more agile for cornering. That's why I prefer the L6 to V8. V8 will have more power but agility is what M3 should run after
Very good point about the E90 M3 vs E46 M3. However, think about the effort BMW has put into to reduce the weight of the V8, I guess it was very costly. And let's c if BMW will do the same investment to cut weight for the L6
Regarding the centre of Gravity, yes, your point is very true