Okay, just gave Sumatra a quick test drive.
#1 - rendering. Unfortunately I don't have any of those "really heavy" PDFs lying around, but it seems to do quite well. Also, while foxit seems to "render as you go" (causing flickering), Sumatra seems to render a full page offscreen before drawing - smooth. On the other hand, it's noticeably slower at displaying the S.T.A.L.K.E.R user manual... slow jpeg decompression, perhaps?
#2 - memory usage. 32meg pdf, Tanenbaum's Distributed Operating Systems. Foxit Reader uses ~4meg private bytes, Sumatra uses ~10meg private bytes. Nothing to worry about, imho. Also, if opening multiple PDFs, you get multiple instances of Foxit, but only one instance of Sumatra.
#3 - stability. I managed to crash Sumatra by having a bunch of PDFs open and paging back and forth in one of Intel's systems manuals... don't think I've seen Foxit crash yet.
Sumatra seems promising though, and it's nice that it's opensource. Might even be worth to dig in myself and run a little profiling on it...