Yes on the gps. No pitot speedo. Turning an "a" impeller to 5500rpm. How fast with a "B" or "BC".

It doesn't really work like that.  You find where your motor makes peak power and then you run the appropriate size impeller to match that rpm.  So if you're already at or past peak at 5500 rpm, running a smaller cut will be going backwards.  Plus you'd probably kill your bottom end.  This is why most stock motors run an A cut.  There power is all in by 4000 rpm usually.

So then is 5500 all you can pull?  If you want to get to 6000, then you could start taking a little off the impeller to get that next 5-700 rpm.  Here's a impeller calculator I took of of Tom's web sight, you can play with this and see.

http://www.jetboatperformance.biz/pages/Jetdrive_Impeller_calculator.html

Yes on the gps. No pitot speedo. Turning an "a" impeller to 5500rpm. How fast with a "B" or "BC".

A half cut will gain you 150 rpm and a full cut will get you 300. i would go to a B and leave it.