What difference can I expect to see if I shoot Ilford XP2 Super at 800iso instead of 400iso then standard C41 develop?
Of the actual 'look' of the negative I cannot say having never done what you are intending.
HOWEVER, at the time of the release of the Chromogenic films the marketing hype was that you could shoot adjacent frames at varying ISO settings (might even have been ASA back then), process normally in C-41 or the dedicated soup, and expect fine results.
If you can get your hands on a copy of the Ilford Monochrome Practice by Coote (http://www.amazon.com/Ilford-Monochrome-Darkroom-Practice-Processing/dp/0240513681
) there is an explanation of how and why this speed range is possible. I can't find my copy at the moment.
There is also an olld discussion thread on Flickr concerning the same issue: http://www.flickr.com/groups/blackandwhite/discuss/72157603523467665/#comment72157603523761021
Like most things I think it is really just a matter of suck-it-and-see. What is acceptable to one man may be unacceptable to another.