Hmm, it may be interesting if you don't change the range of the KPVT.
The vehicles using a KPVT :
All the BTRs - PAI becomes a 5pts killer, the others will probably rush like hell and unload infantry on your tanks, while stunning/killing them with the HMG.
Topas 2AP - makes it a good choice compared to the 2A with tarasnice.
Skot 2A & 2AM - I hope your opponents got lube, seriously.
Soonds good. But it still have only 2 acc and low rate of fire. If other side can do same with AMX-10P, marder, wiesel and FV432 why other should not? It will be some balance.
BRDM 2 - don't use it enough to judge.
UAZ KPV - already a good unit for it's price.
Again it will balance armed recon. Look at replay in first post.
SU-122-54 - nobody uses it anyways.
I use them.
Besides, i'm not sure if high caliber weapons doesn't make more damage, even if they have a disavantage because of the low RoF. It seems to me that the Bushmaster (25mm) is often more effective than the 20mm of the Marder.