I am running mine with a YS 63 on a 13 x 6 apc prop. I found that more pitch and more speed didn't realy help the airframe. I guess with 50 sx try a 12 1/4 x 3 3/4 prop.
I am also running the rx and servos at 6V

I guess the ailerons are about stock, if no a little less than the recommended. I found it a little twichy arount the centre of the movement.
Rudder is as much as I can get.
As for the elevators. Low rate as stock. The test is to start at a low rate and then pull loops while increasing the elevator rate on yor radio. When is snaps out of the loop you have too much travel so back it off a little and that will be safe for normal flying. For high rate I have about 40 to 45 degrees of movement. At this level it is almost unflyable in normal mode, but 70% expo and not to fly it at speed it works out quite well for a 3d setup.

The blenders, flat spins and snaps are to die for.

Good luck

Kris