I tried it on a G51... doesn't work... at least not at the rpm I was running it at (Zinger 12x4). Maybe at 16000 rpm the muffler gives adequate pressure...

Case pressure work just fine. Also very easy to "prime the tank"... on muffler pressure you may have a bit of a problem to build up pressure in the tank before you start the engine, but with case pressure you just have to flip the prop 2 or 3 times on a full tank, and that's it.