I'd guess most -12's went from 0-235 to 0-295 to 0-320....
That's what mine did (the 235 - 295 being before my ownership), so not sure if there's many threads on direct 0-235 -0320.
I believe, here in the States, that almost all the STC's for a -320 conversion require PA-18 tailfeathers....not a big change and if that is all the requirement imposed over in the UK, it's certainly worth it.
One will want to evaluate the engine mount and the impact on W&B and so forth. While my size tends to mitigate the finer points of W&B, in days gone by, for handling if not for staying in the W&B envelope, if flying by myself, it was beneficial to throw a small backpack all the way to the back of the extended baggage. Obviously, this will vary depending on the aircraft and the engine mount selected.
Compared to stock, IMO, the best mods are: 0-320 (with PA-18 tailfeathers), flaps, and -18 gear (I think that's as much personal preference as anything).
The more mods, the more work, weight, and cost (i.e. fuel burn) a -12 is to fly...just something to keep in mind.
FWIW, in my -12, being a mediocre pilot, nevertheless, if I were light and on my game, I could land the -12 (via slipping, etc) as short with the 0-295 as I can with the big flaps it now has. With the flaps, I can be heavier and still make the short landings. Takeoff (even when the 295 was the starting point) is where the real benefit of the bigger engine shows up. Add flaps and it's even more so.