I have an OS .61FX in my Venus. (7.8 lbs)
It is mounted with the plug in the 8 o'clock position, so the stock muffler hangs out the bottom of the cowl.
Plenty of zip for the power grabing outside loop, 1/2 square loop with half roll, and the stall turn with full roll.
The only downside is it's a small plane, and like any craft of that size, it's "flighty", as opposed to "groovy". (compared to a 2meter pattern plane)...croz