11m/s mounts can't be upgraded to 15m/s mounts as of yet. But like the flyers, I think it's possible for Agatio to implement it.
Would be cool to have some of the 11m/s mounts as 15m/s mounts! :smiley:
In pwi, they used something called "galewind spurs" I guess to upgrade mount speed. (which isn't available here) :p