I have built 4 Midwest Magicians exactly like the plan with moveable flaps. Every one has flown very well and almost exactly alike. I do move the CG back about 1/2" or so. The first one had a Fox 35, and the rest have had McCoy 40's. It has done very well for me, as I fly it in both Classic, and PA. I have also flown it in P-40. Mine haven't been twitchy at all, and are the one plane that comes out flat in corners like in the square loop, or the reverse wingover without even trying. It is just a very easy plane to fly. It may well fly better with some mods, but since I fly mine in Classic I have just built them stock. The first three have won me a lot of trophy's in advanced, and I am far from an expert flyer.
I think the big thing with Fox 35's on profiles is that they need to be well broken in. I have built profiles with both the long mounts, and short mounts with Fox 35's, and the ones with the short mounts ran better than with the long mounts. Also, the short mounts did not loosen the joint at the wing fuse as bad. I don't pretend to know the answer to that. Maybe just a difference in wood selection. I also make the tank as solid to the fuse as I can get it for a good run. I have tried foam, and it did not work at all for me.