Mike, I have a GP Spitfire with an OS25FP, and love flying it! Would love to see some pics of your retracts if possible...
Mine is plenty fast for me at this point; I get about 12,800 with a 9x6 Master Airscrew. Tried several other props: APC 8x6, 8x7, & 8x8; the 8x7 works about the best on this motor, and is a little faster than the 9x6, but doesn't have as much pull. Might hafta try a 9x4 like the combat guys are talking about. Someday, if the plane survives my piloting, I'm gonna put a TT Pro 36 in it and really make it scream!