I used a solas to replace a badly worn impeller on my 1996 GTX. I'm quite happy with it.
If your current impeller is working well and undamaged, the difference, if any, will be minimal. You might see a change of 1-2 mph at most on a PWC. On a boat, even less. If your current impeller is significantly worn, then it could make a big difference, especially for acceleration.
You also have a number of impeller options. In general, impellers are a tradeoff between acceleration and top speed. If you want more speed, you can trade acceleration. However, any impeller adjustments are only going to give you a couple MPH even on a PWC: nothing dramatic.
If you really want to go faster, you're gonna need a bigger engine, or you're going to need to significantly modify your existing engine, but modding engines significantly reduces engine lifespan. Also, if you do significantly increase your HP, you will need a new impeller simply to handle the increased power.
