Last Summer I flew the Epsilon with the OS 91 FX stock with muffler and it flew great once I got throws and trimmed good, I was flying the FAI with and it performed very well, not quite as good as 2 meter but many have told me it looked like it flew the same as 2 meter.
You will be thrilled with the OS in it, don't think the YS 63 will do much for it. I also have the Showtime and the problem with it is all of the control surfaces are way to big. Oh cut the ailerons 3" shorter than whats marked on the wing from the fuse end. any question feel free to PM.
Steve Maxwell