I don't know what I've been seeing, but I am fairly convinced I see a less apparent grain structure when I overexpose the XP2. (or dye clouds or whatever you want to call it).
I regularly stay in the ISO 100-200 realm when I use it, and only when I need it due to lack of light, I'll shoot at 400 or 800. All those negs come out printable, even very good, but especially so in ISO 200.
All I can say is try it. It takes one roll. Expose the same object, on a tripod, from EI 50 to 800, print them and see what happens. If you don't want to make a 16x20 of all of them, crop like crazy and print 1/4 of the neg on an 8x10 to see how it looks.
- Thom