Hi
Provia 400 pushes very well to 800. I do it all the time (i hardly shoot Provia 400 at its default speed
) and my opinion is that while it becomes grainer (of course) colour is not visibly affected and image quality remains high. But i use it mostly on stage / performance lightings which tend to introduce colour casts anyway so that's where my opinion is coming from. u may want to shoot a roll and try it for yourself.
u gotta watch out for the shadow areas though if u're shooting in dark conditions (which is presumably why u are pushing film in the first place). The shadows tend towards very murky blacks. Normally i try to overexpose a bit and sacrifice some highlights, if the scene is not too overly contrasty. Sometimes i shoot at ISO 640 and push the film to 800. (i do that more often for Kodak E200 pushed to 800)
if the scene is too contrasty (dark stage with spotlights) u'll lose detail in the shadows for sure. So u may want to consider negative film for those situations - can't always use slides all the time
As long as ur metering is fine things should work out.