There are guys running 10X10 APC with a 55AX on a Viper, the Magnum is at least as "clean" so it should work too. They tried less prop but went back to the 10X10. I think they were running the stock muffler too. With a Macs quiet pipe or with the special order double quiet pipe you should see a 800 to 1000 rpm gain but is the cost worth it by the time you get it to DK?
By the way, did you happen to notice the long flight time with the 55AX
I have not seen the Graupner "Fuel" props here in the USA.
Viper Video W/55AX& 10X10APC:
http://www.youtube.com/watch?v=zdf1E...eature=related