Have you tried the APC 18,1x10, 18,1x11 or 19x11?
I've heard the standard 200 is not the ultimate performer for F3A, but it should be more than capable! Swinging a 18x10 prop at 8200 should be enough... I've never used Mejzlik on F3A planes though.

As for the weight, I don't know the Osmose "personally", but I find it hard to believe that you should have a problem in vertical as long as you stay below the magic 5k.