Swapped out the 15t on my father-in-laws 900ie, (everything else is pretty much stock) but now he said it seems to be slower revving and the pull seems to be flat, and not spunkier.
That doesn't sound right... I would guess there's something else going on. Does he tend to stay high in the revs where the torque curve tapers off?
You could always swap back to 15t to compare, but 14t made a significant improvement for me.