Just got the rsi irons yesterday and use cally 33 ball. I had a sleeve of Nike level 40 so used them for one round and the difference was clear, held better with the same amount of applied spin. Which threw me as i was then leaving myself short pfft . So settle on a ball is what i must do, and have always thought callys at 250 creds are worth using.
It puzzles me though as both balls have 3.5 spin but Nike def held better.