So recently I've been seeing a lot more arms warriors in raids doing really nice damage.
On my rogue I am usually 1st~3rd DPS, I know it's only lolraidfinder but I was just on ji-kun and I have 496 average ilvl, everything socketed/gemmed/reforged correctly and finished the fight on 100k DPS.
He can't have a 476 average item level for that raid, you need 480 or higher to queue for it.
Maybe he was doing the eggs on that fight, you can aoe there, right? aoe is better damage. From what I've read, arms is one of the best dps specs at ~480 item level, according to noxxic.com . It goes down the higher item level they have vs other class/specs. But around raid finder gear, they are 3rd behind frost/arcane mages.
You are a rogue, so I'm assuming you are Assassin. I wouldn't complain about overtuned dps specs, if I were you. However, Arms does really strongly with 0 gear except a weapon. When it gets more gear, a warrior should probably go Fury (unless for w/e reason they got all the noncrit gear).
Things you should also ask/tell was did the warrior flask/well fed? Did he pot? How long was the fight(recklessness/skullbanner is 3 minutes cd and big damage boost, so if the fight was just over 6 minutes, he probably was a bit higher than he'd be if it was 7-8 minutes)? Were you able to sit on the boss during Downdraft (not only can he heroic leap during it, he has double time so he can charge twice per downdraft)?