The quantum leaps I found were getting off the twisty and onto pedals, and getting TrackIR. Those two purchases allowed me to do things I couldn't do without them. As far as stick goes, if you have enough functionality to run throttle, pitch, and roll, flaps, bang switch, and ideally elevator trim, the rest of it is convenience as far as I'm concerned.
It's still the operator, and unless they're actively working at it, most people will plateau well below 'great'.
Wiley.