If your impeller was measured and was a true A and your pump was tight then it would take you around 700 HP to spin that RPM. If you cut it down to a B that should give you a 200 to 300 RPM increase. Add and inducer and that will probably eat up that new RPM. Also, a new impeller is most likely going to bite more than your old one unless your old impeller was nice and straight. A stainless impeller is also heavier than your aluminum one which means it's going to take more HP to spin it.

Do you have a Dyno chart showing your engines power curve?