Yes the TH .75 weighs about 4.5 oz more than the .55AX but the power differance would be quite noticeable. The extra weight vs power ratio would not mean that much on a sport plane but if you are 3Ding that is a differant story.
You will like the .55AX. I have one and it definately has more power than my .46 AX and FX. I am using the .55AX in my Funtana 40 and it will hover at 1/2 throttle.

Cheers
Curt