So it sounds like I am not too much overweight...

I have a suspicion I have done some damage to the engine and this is why I'm not getting enough power - the prop nut must have been loose and it spun off last week when I was trying the engine out in the plane, and of course without the prop the engine revved at an alarming speed... After that the bearings sounded a little loose. Has anyone experienced this? Would it harm the engine? I shut it off pretty quick...

I have a second SC12 (Bought a pair of them years ago for a twin engined Canadair but that's another story.).

I'll try swapping the engines over and see if i get a better result.