I disagree with ZiS 3 "good arty" argument, maybe it's looking cool, but it's pretty much ridiculous for ATG...(Yes, it's still mostly ATG and can't fire like howitzer) ZiS 3 was very fine and extremely deadly weapon, even german engineers admitted that ZiS is slightly better than any German equivalent after examining few captured ZiS3's.
For infantry support better would be use of infantry support gun, pretty much tiny howitzer (baby version of Brummbär and similar to short gun StuG) firing short range like AT gun and with barrage ability like ZiS 3 now.
Best exaple of IFG's are soviet 76mm regimental gun (look very similar to tiny AT gun used in campaign mode) and german Le.iG 18.
Tiny and cute, but very capable as well
Both AT guns really need better stopping power (Yet die horribly if well flanked), especially when It comes to accuracy and maybe bit of penetration and damage buff at very short range.
For example, I get my 2x ATG's killed in matter of seconds by Ostwind who blitzkrieg head-on from maximum range, hit mine, get AT naded... and still be able to flank both guns, because from 4 total shots any of them ever hit the tank.
Same situation with Pak 40, where I easily flank and destroyed few PAK crews just by charging T70/T34 head-on on the gun and don't get hit - I think that needs to be looked on.
In CoH ATG Has some area denial (partially psychological) effect against armor, because You didn't really want to face ATG's barrel, In COH2 I don't feel any serious danger from ATG's side.