Well, you shouldn't ever use Lee Sin as a reference point for balance. Lee Sin gets 350 MS because Lee Sin is Riot's love child. No part of Lee Sin's kit calls for greater mobility than the astounding amounts it already has.
Anyways, what you're experiencing is counter-niche balancing. Champion has some super special niche, often scaling, and because of it, is nerfed in the affected stat early on to balance it. Think Orianna's base AD being like 41 to account for her clockwork passive, or Sion having the lowest base health in the game (I think even lower than small gnar) because he infinitely scales on it. Kass is one of the few, if not only, melee champs in the game that doesn't get scaling MR due to his passive 15% MR. It -used- to be a factor on Galio in that he didn't get scaling MR either because he uniquely used MR and would stack it.
Riot likes to makes awesome niches for champions and then nerf them in the same stat for it. Some people like this design. I find it abhorrent. Lazy, really.
So, since WW heavily scales on MS, he gets shit MS. Be glad he has 335 and not 330 or 325.