I've shot Fuji Press 800 at box speed with decent results, but I've found EI 640 to be pretty much ideal (decent balance of speed, tight grain and good tonality). I also think this film could be a tad more sensitive to processing variables than some other films. When I was in charge of in-house processing at a photo agency, I regularly ran FP 800, pushed one stop, through our Noritsu (which I adjusted–i.e. slowed down–for proper push processing, also with okay results...we actually thought pushed 800 was better than the test rolls of FP 1600 we were given to try out in 2000 for the US Open).
I think exposing at 400 is going a bit far, but the best thing to do is test. As was discussed (and argued) in another thread, it's a matter of knowing your film, camera and meter for getting the results you want. Burning a roll or two in the name of drawing a bead on your "ideal" exposure value is time and money well-spent.
- Barrett