Stick with the Acromaster stock. The newer engines are much more powerful than the old 15's, and a Medallion 15 is old. I'm not surprised if someone is using a 7x3 prop on an OS, that it flys like a Goodyear racer or a quarter-midget - it's probably turning 20,000 rpms. Use an 8x4, 10% nitro, and 0.012 x 52 foot lines (the 15 std for 50 years), and you should be fine. Don't worry about extending the wing - that's another reaction from the more powerful modern engines. Fly it stock.

I also agree about the flaps - they are almost more trouble than they're worth in a 15 size, and will make very little difference in the whole scheme of things at these sizes.