Sitirus does not work for Swampert, sadly. CBTyranitar will 2HKO the standard Swampert with Crunch with Sitirus recovery or not.
I think even if there was item clause, Tangrowth should have Leftovers. If you are using it as a Gyarados counter, Leftovers ensures that it survives(Its a very close margin, if it taunts you, you can't Leech seed or Sleep Powder so your recovery is very very limited without Leftovers) Power Whip is about a 3HKO, and DDGyarados with 383 Atk 3HKO's with Ice Fang without it.
What are you using on your team that needs Leftovers? It depends. Things that depend on substitues, like Raikou, probably need Leftovers, and should have priority. Things with a recovery move probably should drop Leftovers(Blissey and stuff). And things like Vaporeon sometimes work better with Sitirus berry(25% recovery is nice).