Hi Probuster,
I had an Xtra Easy trainer, in fact it's still in my garage. I had an OS FX .46 in it that didn't overpower it at all. The only complaint about the trainer that I had was the high amount of downtrim I had to put in it. The plane just kept rising forever. I ended up putting small shims under the back of the wings to lift the rear of the wing up a little. I used popsicle sticks and ended up using two of them. The popsicle sticks did get the plane to fly level for me, but you may want to fly it first before you start adding shims, just bring the shims to the field and some CA and add shims between flights until you get it right. Other than that, it was a fun trainer, it was highly acrobatic for a flat winged trainer. Because of the huge wings, and the fact I get bored just flying in ovals, I used 12 rubber bands to hold the wings on every time I went up. I never lost the wings, even when I was pulling incredible G's. I'm not familiar with the MDS .40, you might want to get an Evolution .40 instead. I think it's about $80. I got the OS Max FX .46 because I knew I'd want that engine for any future planes, and that engine was $109. I liked the OS because it pulled the big Xtra Easy around real well.