I used extenders on bodysuits and vests. They're basically just a strip of material with poppers. They were generally good although we did come across a couple of vests where the poppers were different and we couldn't fasten them.
It might depend on the size of your baby though, we just needed them to stretch over cloth nappies but if you have a chubby baby you might find that the chest gets tight before they outgrow the vests lengthways.
We are using them for our DS at the moment as his torso is ridiculously long! The next size vests are too big on the shoulders so it means everything fits better for longer so worth buying them as the extenders aren't very expensive.
They wouldn't work on sleep suits. They'll really only help if your baby is growing out of sizes because he's long (so chest, arm length, shoulders, etc still has lots of room) or you need to go over bulkier cloth nappies.