Take your pick:
https://www.coh2.org/topic/103324/state-of-the-sturmtiger/page/2#post_id797856
Reverting the mutual exclusiveness with the Tiger II is not unreasonable to me, I don't really agree with that either (wasn't on the team yet when that was done).
XP is already way lower than the only comparable unit, the AVRE, at 1400/2800/5600/etc vs 2020/4040/8080. Furthermore the Sturmtiger can have an easier time gathering veterancy by hitting vehicles or even scraping them, because it deals a lot more damage to vehicles than the AVRE does. Not sure why you think it should be even lower.
Its vet 1 ability was just changed to something much better.
I don't think bringing back world piercing is a good idea. The game needs less cheese, not more. It'd be a poor solution that only benefits the user, while making the experience for the receiver significantly worse.
A casemate tank firing a huge rocket is simply never going to be able to be competitively viable (without being overpowered). It's an illusion that it could ever be balanced. It can't. It will always be either overpowered or underpowered, just like similar units like the B4. Relic should have never put a unit like this in the game, but we're stuck with it now. The least we can do is making sure it's at least good enough to use for fun in (casual) automatches, which it is right now as long as the player knows a thing or two about how to aim it. And it'll get a bit better with the added utility and power of the revamped vet 1 ability.