Mogg War Marshall does seem like a good addition, I'll have to give it a try.
I feel your misgivings about
Warren Instigator. My thought was that with the buffs from
Goblin Chieftain and
Goblin Bushwhacker it'd be a 2/2 or 3/3 double strike which seems pretty damning, but does require a few cards to get it to that point. I believe I've played all of one game where he connected consistently to get more goblins down, so I may take your advice here and switch it with War Marschall or
Goblin Wardriver.
Goblin Grenade does pretty well in my experience; swing in with goblins then sac a token for 5 more damage. Sure it'd be signifigantly better as an instant hehe but it's still pretty good I think.
Thoughts on a Gobin Guide playset? A 2/2 haste for 1 seems like a no-brainer in this deck I'd assume? Never used it, is the extra land to your opponent make it not worth adding?