Ok, we agree that Heatran is a bad switch in against Hydreigon, so why worrying so much about Earth Power?
My Heatran too runs lefties, and I would never switch into Hyd if I don't know each exact move he carries. Not only he's got Superpower, but Surf, Earth Power, and even Focus Blast. So me, as a Hydreigon user, would never expect him to switch into my dragon. And as I said, Surf allows to hit hard many many things more than EP. I mean, between fighting moves and Earthquake on other teammates, I don't think that you have to run EP on Hydreigon just to kill Heatran.