The Med. 15 is plenty of power, maybe even a tad overkill. My buddy has an Akromaster with an OS .15 on it and even with a 7x3 and a slightly rich run it goes like a bat from hades. Currently on 52' lines, gonna try 60' next time out.
I'm sure a Medallion .15 is on par with the OS .15, and probably a bit lighter. I've had a Ringmaster Jr. with a Medallion .09 and it flew fine, the .15 on a slightly bigger / heavier palne should have plenty of power to do the pattern.

One tip I've read about and plan to do to my Akromaster kit when I build it is strech the wing by not cutting the excess material from the leading and trailing edge stock. You would have to make a couple extra center ribs to use the original bellcrank mount, then keep the standard spacing for the rest of the span except the tip ribs would be moved out to the end. That makes the last bay an inch wider, plus the extra inch on each side in the middle gives you a total of 4" more wingspan. There is a thread here about it, or maybe on SSW.