Lolwat? Of course Moltres is Timid. You do know SR is included in these calcs? And assuming low rolls makes it a 3HKO, you still have to avoid Air Slash flinch.
Timid has no chance to 2HKO Special Sweeper Grumpig with anything, so I was assuming you meant Modest. Plus, if I were the Moltres player, I'd rather not risk a Twave or allow him to get any CMs up before I can respond with the appropriate threat.
Also, I don't include hazards in all of my arguments (unless I'm lazy and want to say something is OHKOed at ~87%), since I can say from personal experience that they're never 100% guaranteed in any given match, especially not in UU. Of course, Stealth Rock plays a considerable role in all of competitive Pokemon, but if you have any experience using Pokemon especially weak to SR, you'll notice how it's not really all that hard to keep off the field for most of the match if you even bother to. Taunt, Rapid Spin, and offensive pressure can often leave your opponent without a single spare moment to set anything up.
Or I can just shit on you with Spiritomb since you're running lolPsychic/Power Gem and sweep later with your "special wall" out of the way.
lol yeah, maybe if Grumpig were my
only answer to Moltres. 90 base speed isn't exactly fantastic, so I can still fend it off with a good scarfer, Arcanine, Houndoom, or almost anything else faster. Also, I've played against a Sucker Punch/Pursuit Spiritomb before. If they stuff up their prediction, Grumpig can switch out and live to wall another day.
You probably underestimate Grumpig because you haven't seen it in action, but I've have decent experiences with it, and having the ability to counter Moltres, support your team, and still be decently fast all at once opens up a lot of venues for it. You should try it out.