That's impossible to answer. Performance depends on EVERYTHING working together. You can install a 24mm carb, and 400.00 pipe that will do nothing if you belt is too short, or the wrong spark plug is used.
Strictly speaking if all is in good working order, and everything is know to be within spec. engine performance upgrades HAVE to work. It's a physics thing
It sounds like the fella did the right things but maybe never got it balanced correctly. You can throw a pipe, and carb that are supposed to add 2-3 HP, and have the bike not even start. It's all got to be done right.