You have it right. I have a bit of experience with the 55AX. I have found that I can count on it to deliver 12k with an 11-7 prop using 15% nitro with the stock muffler. Using a Mac's 8.5 muffled pipe, I can squeeze about 13,800 rpm, although with that pipe, the needle is out about 3-1/2 turns. That pipes is too small, even though it is supposed to be the "right size" pipe for that engine. Is is pipey, overly rich in the mid range, and sucks a huge amount of fuel.

However, with a Mac's 10cc Quiet pipe, the results are far more pleasing. I got 13,500 average with an 11-7, and the needle was only out about 1 turn from the muffler setting. Consequently, the mid-range performance is linear, and the fuel consumption is reasonable. Having to open the needle on a piped engine is normal, as the engine uses more fuel when it is on the pipe. Common sense.

I find that the .55AX is not a real replacement for a brute .61 (Rossi, Webra, etc) but is equivalent in power to many .61 engines from the old days. It is ideal for aircraft like the Mach 1, or your P-5, where the extra weight (higher wing loading) is not worth the power trade off.

The deciding factor between the AX and the Evo would be power output, being that they weigh the same.

-Robert