US Engines is not going out of business in fact they have just launched a new website and have joined with the Quadra line of engines. I have just ordered a new Quadra 52. great engines! see their website at www. us-engines.com.
The US41's I have will turn a 18x10 prop at 6800 rpm and are not completely broken in. Hans at US engines beleives 7000 rpm should be attainable. I would say the Engine should fly most models up to and including 22 lbs ( my opinion) so much depends on wing loading etc.
Regards Brian H.