I don't think there's such a thing as "too fast" for the mythic race, since that's the entire point, but I will say that I think Heroic is a bit easier than it should be.
My raid is already 13/13 N & 6/13 H, and after we lost several people toward the end of BRF (we have exactly 10 if everyone shows up) we no longer have the members to do Mythic even if we wanted to. For people like me, I don't think this tier is going to have the lifespan I'd hope for in a last raid tier.