Urgot was overbuffed but Q nerfs seem like a bad way to try and bring him back in line

Urgot's Q is already considered to be underwhelming. If you nerf the slow on it( the only reason to use it outside of lane) then it will basically become a dead ability. I think you should look at the leg cooldown. 2.5 seconds from lvl 13 onwards is a bit too low. 4-5 seconds seems more reasonable since before the buffs, at lvl 18 with 40% cdr each leg would have a 6 second cooldown.