Yasuo needs scaling MR, Riot.
You claim that he performs best mid, yet you don't give him any scaling MR to help him get through his (Rather horrible) laning phase. To quote CertainlyT:
"Yasuo's shield gives him a substantial advantage against champions who need to expend resources to break it. Removal of MR gave them a better matchup -- mana based damage is more often than not magic -- without having to resort to solutions like 0 health regen."
I don't understand this reasoning at all. Yasuo's shield is popped by literally any form of damage, be it an ultimate to an auto attack, it's going to go off. No one has to expend resources to proc it, so why doesn't he get any scaling MR when his main lane is filled with mages?