Granted, though, Galbatorix has personally killed enough people and made enough Name-slaves that the spell would cause him ultimate pain however his role as high king played in or didn't.

Very true. I don't think Galbatorix is anywhere near the complete monster Paolini tries to portray him as, but he's certainly done some evil things. Primarily the mindbreaking.

One thing I do know, though, is that if Eragon ever got hit with the Empathy Spell, he would get wrecked. And that statement is very much rooted in canon. Because canon includes a certain character who feels lots and lots of pain, which Eragon is explicitly responsible for.