Over line the TFO or just buy an Xi2.
I agree. I was having trouble getting my "6wt" Axiom TFO to cast 7wt GPX floating line. I was getting very frustrated, because it was casting the 250 Grain sooo good. Once I bumped the line up to 8wt, I was casting great. I recommend bumping up the line until you find the right one.
I think they only make the Axiom in 6, 8, and 10 wt. But it might be worth a shot looking at the 10wt.
Travis