my ax55 was/is unrealiable in the midrange (havnt tested it in a while), i got it to the point where it doesnt die but it lags so horribly in the midrange that its no fun for 3d. i replaced it with a gms .47 that was well broken in and much more fun to fly when you know a engine isnt going to crap out on you, and that when you give it throttle it actually responds instead of lags. imo i can hardly tell a difference between the ax55 and the gms .47 in terms of pull out both engines rocket the mojo out of a hover. the ax55 just a tiny bit faster than the gms but not worth the unreliablity and price (both turning 12.25x3.75 props)
my advice is get a engine that you trust. not a new one, one that has had some time on it one that you will bet $1000000 that it can last the whole flight, if you cant trust the engine on a 3d plane it is no fun at all, you worry about the engine not the manuver. it was such a relief when i was flying the mojo with the gms going into a low altitude hover knowing the engine wouldnt die/sputter.
if you want to use that ax55 put it on the plane u fly most (non3d plane) and have at it, after 5+gallons and your trust throw the ax back on the mojo and have fun.

i would get the mojo .40 its not that hard to build