Just to be fair, the OS91FX can perform pretty well, when propped correctly, and it really needs a tuned exhaust.
Configured like shown here, it runs pretty well.
http://www.rcuniverse.com/forum/fb.asp?m=4564450
and it can produce some good power.
This one here has a jett-stream muffler, a good fuel system, and an APC 12x8 on the shaft. Look for between 12,000 and 12,500 rpm on the ground, and it will unload nicely onto the pipe in the air.
Most guys over-prop the heck out of the OS. Dave nailed it - it is just a "big" .60. For most sport/speed use, a 12x7, 12x8, or 11x10 is the most you would want on it. Drag it down much below 11K rpm and it is dogmeat (without some help from a pipe or something)
The Jett 90L is pretty well documented here. Generally prefers a bit less pitch, 12x6 typically, but runs about 1500-2000 rpm higher on the ground and unloads a ton in the air.