Though unrelated to this thread, I have put the 55AX on a Reactor 46 with a 12x4 and it "screams". I think the 13x4 is more appropriate if its 3D you want. 12X6 is also fine.

The 55AX I have on that Reactor is able to hover the Reactor easily at 50~60 % in what little hovering I was able to do in January. It may not have sufficient power for the more radical moves on the Reactor. But the Extra 300SP is smaller and the 55AX should definately be sufficient with 5-10% nitro. I have one awaiting its turn in the box

Ameyam