We might give Urephons a speed buff indeed. However, I think it would be better to have him slightly slower than Manticores. This way, it's more of a strategic decision whether to use Urephon or not. Using him will make the Manticores more powerful, but also give your enemy slightly more time to react.
Of course, this change makes sense only if we allow heroes to be used without transport boats (otherwise, the boats will be the limiting factor and we really don't want to change their speeds).