There is a Seagull Decathalon at our field that WAS flying with a .46-AX. It HAD plenty of power. Unfortunately it went in a couple weeks ago. There is also a Phoenix Decathalon that shows up now and then with a Thunder Tigre .46 and it has plenty of power as well. Another guy had a kit built GP Decathalon at the field on Saturday with an old .46 and performance was unbelievable.

I can't imagine your plane being underpowered with the .55. Is then engine running right? You might also experiment with other props. Try a 12x7, 12x8 and even an 11x8. My nearly 7 pound GP Revolver with a .55-AX and 12x7 is a bullet. Of course the low drag airframe has a lot to do with that. I run an 11x8 with the .55 on my WM Rambler. Why 11 inch? Ground clearance mainly. The higher pitch keeps the RPMs in more of an optimum range. Remember... The .55-AX has a much different power band than the .46. it develops power earlier and has more torque for spinning bigger and more aggressive props. These props mean more thrust. I consider the .55 to behave somewhere in between they typical 2-stroke and a 4-stroke.

I wouldn't bother installing a larger engine, it will just make things that much harder.

Just don't get fooled by the Decathalon. Yes it has a big, high wing which may make people think it's docile. It has a stubby little fuselage though and can be tossed around pretty good. I've seen people get into trouble with them. Most often that trouble seems to come from poor ground handling on take-off and stalls caused by low airspeed on landing.

Honestly, I'd recommend getting some other plane for the time being. Try building a GP Rapture over the Winter. It is an excellent flying plane. A 4-star would be another excellent choice to build. If you don't want to build, check out the World Models T-34 or Super Sports, GP Easy Sport, 4-star, Tiger-2, or even the low wing trainer from NitroPlanes. None of these are scale looking but they fly great and won't get you into trouble.