Some are barely noticeable, while others are very obvious.
Especially knowing what your motor has to offer.
Biggest mistake in my eyes that people make is they instantly want to start throwing hp at the boat with bolt on shit that is only gonna net you 20-50 hp at the end of it all.
If your pump is worn out, you'll never see/feel 20-50 hp.

Anyways, I did a simple rebuild and clean up on a pump and from the driver seat he didn't really notice anything but his wife says the boat pulled way harder out of the hole on her ski.
Nothing really changed as far as hardware.
Just tightened up tolerances and cleaned up the edges.

-beerjet-

7 mph more is what i got out of me old pump after jaimie brock and i tuned her up and .500 more rpm 

Pump rebuild as well bolt on shoe/plate, 9 vane bowl + stuffer, inducer, diverter and snoot... Out of all the gains like out of the hole, crusing...ect. I like wot with the shoe /plate the best.

Over and above a properly clearanced pump, the most bang for the buck is going to be:

1.  Detailing the leading edges of the impeller
2.  Installing an inducer
3.  Bowl work.

We have found that a stock Dominator bowl (newer design) is roughly equivalent to a fully prepped Berkeley bowl.

The pump is only part of the equation though.  Setup is equally important.

Cheers,

Joe