Ok, which handling differences (that are modeled in AH) are not a direct result of engine or armament changes (weight, bulges, torque, CG etc.) ?
If you're talking about going from the G-2 to the -6, there are none. The G-6 is a G-4 with MG131 cowl guns, the G-4 being an improved version of the -2 (bigger landing gear etc.). They both use the DB 605A engine in, from what I understand, the exact same configuration. However the difference that this causes between the G-2 and the -6 is significant. The G-2 is faster, climbs better, and turns better. The G-6 has a noticeably heavier armament, is more docile, and the version we have in game has the
Gallandpanzer head armor which provides a significant improvement in visibility (more than the Erla Haube canopy IMO).
The differences between the G-6 and the -14 are even more pronounced, with the G-14's MW-50 boost system providing superb performance on WEP (though the added weight makes it the slowest Gustav on MIL). The G-14 also incorporates the Erla Haube canopy which again improves visibility.
You can't really sub one in for another, and all of them represent an aircraft that played a significant role in significant numbers.