I have a set. The underdrive isn't even noticeable. Its slightly noticeableat idle on the headlights, they get a little dimmer. And the reason I said why would they run hotter was because they dont, not that I dont get the concept.
I have them on both of my stangs, and noticed NO difference in temp. My volts register slightly lower, but it is not a problem at all. They are a great mod, definately worth it. I would reccomend going with Steeda