Baseline is GV damage is easier to fix than aircraft damage. GVs are more rugged and can handle 'manhandled' field repairs better.
anyways, im sure some aircraft damage is way easier to fix than replace a turret on a Tiger.
and if you can fully repair 4 cannons on a wirbelwind in less than a second, is fixing one or two 50 cals on a pony unrealistic.
I agree completely on the first point. If the damage is severe like main gun or tracks or engine the GV should not have an "instant" repair. I don't see anyone willing to wait 30mins for re-tracking a tank (im guessing here cause I really have no experience with tanks) so in game maybe damage repair for GVs should take more time.
IMHO time should be allocated on basis of damage on GVs and even the rearm/refuel for aircraft should depend on amount being refueled/reloaded. For e.g. if loading 25% fuel takes 30sec, then 100% should take 2 mins.
As far as the turret on the WW/OSTI goes, killing the turret isn't damage to guns. It simulates killing the soft pink body of human flesh operating the guns. So when you resupply, its not the guns that get fixed, but that M3 or SDkf (more appropriate I guess

) just brought you a new gunner.