That is pretty impressive for an engine design that is at least 20 years old! I don't think you will get much more out of it with a larger carb as the 50 never really did turn a lot of revs compared to the 40s and 45s.
Does 12,800 on a MA 11-6 with 15% nitro qualify as a lot of revs? Don't know where you got that info. I think Fox advertised something in the upper 14's with 5% nitro and a 10-6 prop at one time, which is about the same as the present .46 ABC. Flew my Big Stick .40 fast enough to rip a corner of the covering off, the adhesive was still stuck to the balsa. Climbs straight up with an APC 11-5. Forgot to tach it though.