Ionization energy and ionization potential are used interchangeably in my experience. If you want to argue for differentiating between the two, be my guest, but chemists can be awfully slow in adopting new terminology standards (I still call ethene "ethylene," after all).
I presume that you've seen the spherical harmonics plotted for the higher-energy shells. (They're on Wikipedia somewhere, in case you haven't - I remember seeing them not too long ago.) It's not just that they're farther from the nucleus in a general sense - the probability of finding an electron is noticeably smaller nearer the nucleus for, say, an electron in the sixth energy level, relative to the second energy level.
There's probably also some benefit to reducing electron-electron repulsion by knocking out an electron further down the groups, although that's kind of a handwavy thing to say, and I can never remember just how well it holds for heavier elements in all cases.