cant get RPM readings
What size is your carb bore? It should be around 9mm. That's the Q500 carb, which is the one I have and can peak 23~23.5k on the ground with using a 7x9 prop. The venturi bore is 11mm, which will give you a little more power, but think about it: that's like driving a ferarri with the speedo buried all the time. Not exactly a smart idea.
If your carb is only 7mm or so, call MVVS and ask for a modified Q500 carb for your GRRT. It may take a couple weeks to get it, but it's WELL worth it! Also make sure of the obvious--is the carb opening all the way? It can be really hard to tell with the rear-induction.
Carl
PS- What fuel are you using? The GRRT is an FAI pylon engine, designed for ZERO nitro. I run 5% but with a shim or 2 to keep it happy. Make sure you are shimming it enough for your nitro content!