Actually HEAT doesn’t affect usf that much since their tanks will almost never bounce anything anyway. IIRC the increased damage isn’t enough to let the 160 damage tanks (p4, panther, jp4) kill mediums in one less shot either, so it’s not a huge deal vs. shermans and Jacksons. I think it reduces the amount of hits that the kt takes to kill mediums by one though, and lets the puma 4 hit mediums like a tank (which is really funny and useful actually).
HEAT gives 30% more damage, so (almost) none of that is actually true, unfortunately. Mediums will still take 4 hits to destroy mediums. But the KT still takes 3 shots to destroy mediums, and the puma takes 5 (instead of the usual 6) shots to destroy mediums.
Basically, HEAT only usefully changes the shots to kill if you're using a vet 2 or lower puma, if you're facing a light vehicle (not that you want to use HEAT in this situation, so I guess some might not even call this useful...), or if you're facing a premium medium or heavy tank.