RE: Q500 and a OS 25?
If you're looking for a "fun" class, check out the pylon forum for the "Club 40 Racing" thread. There are 2 airframes approved and use .46 sized engines for a cheap investment. It's anything but "boring" (sorry Dave). It's definitely not exciting as Q500 or QM, but it's a great way to get people involved with racing.
The SkyRaider w/ a TTPRO .46 will be in the mid 90 mph range, which is plenty slow for a 400' short course. The old guys will definitely be able to keep up with it, but it's still fast enough to have fun.
We have a series here in Central/South TX that's going great. We have anywhere from 20-40 pilots per race and have a blast.
Tim