RE: OS 50sx prop size
I have a 40 size MOJO which is VERY close to your katana. They should weigh about the same and are intended for no holds-bared 3D. On my mojo I run 3 props depending on what I can get.
I prefer APC prop because I hit the ground a lot and even when the get shorter I just keep flying for the day.
My favorite APC's are
1. 12 x 4
2. 12.25 x 3.75
3. 11 x 4
If they spool up a little slower than you would like try a master airsrew or preferably a wood prop like a 12 x 4. The wood props spin up MUCH faster although they give a little less thrust in the end I think, from flexing.
Hope it helps,
Dave
Hey try the tower hobbies .46 muffler, its a gem for 15.00 bucks