Brainstorm is superior to Impusle because of the second mana requirement. In a format where the crucial turn is on average 1.5, the Impulse puts you behind on the race for virtual card advantage/card quality even though it digs one card deeper. I don't think the trade off one card deeper for one turn later is justifiable. I've tried playing Impulse for a while, but every time I draw it, I'd rather have drawn a brainstorm.
I don't think the avergae crucial turn is going to be the same in an unpowered meta. I've always loved impulse in monoblue. The deck really wants a better selection of cards and Impulse does that better than Brainstorm.