I've been riding the group bomb wave for about a day now, it leveled up on all bombs a few times yesterday.  The bombs with the higher payouts obviously rise faster after each round, but I think it will be successful for at least a few more rounds since it's in groub-bomb mode, the small required buy-in makes it pretty easy.  I'm sticking with the top 3 profit bombs right now (50%, 35%, 25%).