Let me fix that for you - the adds don't have aggro. They fixate on the closest target. They are not a threat before the scream. For when the scream occur u need to stun the adds and continue to
cleave them. I am bolding the cleave part. Cleave, not aoe. Garosh damage is more important.
Now on the topic
Arcane is god for the 1st phase with the adds and for 2nd phase if your RL let you stay in the melle and just burn the boss. Glyphed CoC+Fire bomb+Arcane Barrage cleave can do a lot of dps. I personaly have reached 1,5 mln burst... anyway. But arcane is very very bad for the 1st intermission. It does bad dps on the adds there and that is problem, because they have to die asap. Even with banking and CD usages i have sometimes forced myself to use my second pot there just to make sure we can deal with first intermision (if that is done its basically a kill). Well ofcourse that is fixable with smart group split, but still it feels bad to be the bottom of the dps on these adds
With proper fire gear you can do good damage on Garosh. If u manage to get good combustion when the adds come and u manage to spread it on them you can do some sick burst. But then it comes again the first intermission and i believe that fire is even worse than arcane there, especialy on 10 man.
Frost is the best choice by my opinion for that fight. Banking procs for the first set of adds and then frozen orb for the second set and you can top the damge done on these damn adds. Using bombs on the desecrated weapons to get extra FFB procs for 2nd and 3rd phase is also awesome exploit to get more dmg on the boss. The only bad thing about frost is that its nature is he can only cleave. You can't AoE the adds in first phase properly with frost as the other two specs can. And as i said when they fixate on people and healers can handle it people start to die.