The 12.25 x 3.75 is great for hovering, but the throttle response is better on an 11 or 11.5x4, I tried the 12x4 (all APC's) could not tell the difference from the 12.25---
I run the Tower muffler ($13 or $14) on my 46-FX's for about 600 RPM's extra and a better sound!
Have fun and keep us posted-
james