op if you wanted to buff his w, id do it like this :
0.3 per stack until 15 resists (at 50 minions instead of the current 60)
0.2 per stack until 30 resists (at 125 minions instead of the current 120)
0.1 without cap (makes 50 armor/mr total by lategame and 320 cs)
if you just doubled the cap, it would already increase his resistances by an extremely high amount by midgame.
the way i changed the scaling would slightly buff it early game (where he could use a slight buff right now), have it roughly be the same by midgame, and a decent buff by lategame.
if youd really powerfarm 420 minions, youd get the 60 armor/mr from the passive in total. however in most realistic games the change should give him between 10 to 15 resistance points more by lategame.