If you up the nose wheel 1/2" in diameter you'll gain more than upsizing all the wheels the same. But she may want to keep flying unless you slow and flair well on landing. I'd probably opt for the 12" myself, though I am running 10.5 X 8 and 11 x 7 on my ST G-51 engine. You won't need the additional speed to gain lift with the .55 on an LT-40, though.