I got my Toys r us mp Grimlock a few days ago. As far as his hips in robot mode are concerned I think they're just fine. He is top heavy. MP Optimus top heavy too, how far can you articulate him leg and hip wise? I believe MP Grimlock holds up better in carrying weight on his legs.
One of the keys to balancing him I think is that his upper body swivels with his crotch (Oh get your mind out of the gutter) independent of his hips. You can have the right hip swiveled one way, left hip swiveled another and crotch with upper body in yet another independent of each other in the horizontal plane.
As far as lateral hip movement. I wouldn't have him doing splits or standing on one leg so I think he has fair amount balance there provided you adjust the knees the leg swivel and don't go too extreme with the weight distribution to any one side.
MP Grimlock can get a decent range of poses. I don't expect my Masterpiece transformers to dance. Do you? http://www.youtube.com/watch?v=EtGQgSY9Nn4