The two cars are not exactly the same between AC and GT7, the one in AC is a '
14 and the one in GT7 is a '
17, the '14 has a stiffer suspension set-up than the '17 (which in turn has a stiffer set-up than the '20).
I've just taken one for a drive around Brands in AC, and the only way to get 'mad understeer' was to go into corners far too hot (which is easy to do, as it's stupidly quick for a road car), getting the rear active on 2 gear corners wasn't an issue either, and certainly not like an open diff, which would have seen power spinning away, and I found it flows perfectly well, in fact for a car of it's weight it's stupidly agile.
Edited to Add:
Stock car and track, no mods for either. TCS is either off or at its lowest setting, ABS, but no Stability Control. I even left the tyre pressures stock.