1. A Menz Standard 18x8 will fly a 12lb plane in 3D - it is dependent on thrust at low rpm where most 3D is flown and weight. Max rpm has nothing to do with it.
2. As well as they fly, C-ARF's planes tend to not be lightweights, but they are touting 10.5lbs and up for the 2X2. You can be sure that this is with a glow engine and CF pipe, Lipo batteries, etc. Best to watch the first to actually get the plane in the air and see what weights they actually end up with.
3. Your muffler choice is going to hanidcap your quest from a weight standpoint.