If you push Astia 100F two stops, then the apparent contrast in the final results will appear to increase. Pull processing does not seem to have quite as an effect, though I only tried it one stop. I found that Kodak E100VS does perform slightly different pulled one or two stops, though exposure needs to be very accurate. The best choice I have found for push or pull processing is Kodak E200, though it is normally a medium to low contrast film. Newer Fuji Provia 400X pushes very nicely with only a slight change in contrast, though I am currently trying a roll with night shots set to ISO 3200, so we shall see what happens.
Even after the Kodak reps gave me some rolls of Portra, and suggested I try higher push settings with it (as an alternative to E200), what I found was that it simply was not working as I wanted. I think there is some room to push or pull Portra 800, but I don't particularly like the results more than E200 shot at similar ISO settings. Remember that differences can be very subtle, though sometimes it becomes very apparent upon comparison.
Ciao!
Gordon Moat Photography