We do not need veterancy bonuses.
It's nice to know how much mass each unit has killed so you can track whether it was efficient, or for sentimental reasons. We can track that info (at very low CPU cost) without giving any bonuses.
Removing veterancy bonuses from ACUs would affect balance. It barely affects the balance of anything else.
We could also remove the instant HP boost to ACUs. We could keep the regeneration rate boost but not give free hit points to the ACU when it goes up in vet. For example an ACU with 10,000 max HP, 2000 current HP gets its first vet, it has 11000 max HP, it instantly moves up to 3k current HP. That extra 1k hp could help it to survive. If instead it went from 2,000 hp to 2,200 HP with slightly higher regen, veterancy would still be important but it wouldn't have as much immediate impact on the game. Or we could just fiddle with the balance so ACUs don't get any veterancy bonuses at all.