15t is the max up front.  There are limits to how fast anyone can dune any bike.   Up, down and lots of turns require a wide powerband, lot of shifting, good SUSPENSION and lots of balls to go fast when your not 100% sure whats over the next lip.  With 14 x 40-42 3rd and 4th gear duning is moving pretty good.   Tire rollout plays a part there too.