Haha- my 85/1.8 is on its way now. Good choice! ;-) (that said, I'd have gone for 135 if I had FF, it's just longer than I need on APS-C)
Anyway, I digress. I've not used either of those particular 70-200 lenses, because I don't typically find myself needing IS at those lengths (usually using them for sports, high shutter speeds). But everything I've seen with the 2.8 IS ii is cut-you sharp. Not that the f/4 is a bad lens, but the 2.8 ii is just so sharp, and fast to AF, and doesn't degrade as much with extenders... So while you are correct, you'll be able to use higher ISO to compensate for the stop in aperture, but you'll lose the DOF control, and I dare say a bit of IQ from your lens... Just my impressions.