Well, M-42 is cheapest AT gun
doesn't make it good or cost efficient.
If T34 had same MGs as AEC, I wouldn't complain.
medium tank need a good main gun. Shoot and scoot are much more important for allied tanks. You don't really have the option of standing still when you're kiting schreck.
Faster firing medium tank in T4, that means, that it comes in late game, against such tanks as Panthers. It won't be usefull on that position neither with slow gun or with fast. It still has bad AT and AI stats.
the t34/76 gun is pretty crap to begin with, there's no shortage of needed buff. It's got bad scatter, bad reload, and the worst pen of all the medium.
However, if I can only apply one buff, it would be to buff the reload. This is because buffing the reload will affect the tank's anti-tank and anti-tank infantry. buffing the aoe will only affect the gun's anti-infantry capability and buffing the pen will only affect the gun's anti-tank capability.
Since the gun is pretty crap at both currently, I would buff the reload, which will make the gun better at both anti-tank and anti-infantry.