Funny that I'm the guy that thinks that gel spun backing is perfectly safe but I think you would be CRAZY:eyecrazy:
to flyfish with 40# tippet.
Fly lines are not that stong, backing is not that strong(50# gel spun has terrible knot strength), you rod is not that strong, and you(or anybody for that matter) are not that strong. But the real reason I would recomend against it is that the possibility of loose line looping around something as the fish gets on the reel in the blink of an eye is more than a little scary.
I don't think that 20# tippet is too light for fly gear, and I like my tippet to be the weak link.