I have pushed it up to 4 2/3 stops, though it does not push in a linear manner. Exposure compensation is needed for successive pushes. If you take a look at some of the night shots on
BigTimeOperator.com website, you can get a little idea of how it responds.
Usually I just use it at ISO 800 (push 3). The grain and contrast really undergo almost no change at that level. Beyond that there is a tendency towards a blue shift, and slightly more contrast. Grain does not become more noticeable unless you underexpose.
Hope that helps a bit. If you have any other questions, or want to know some suggested settings, I can post another reply. I think it is a situation that you have to work with your lab a bit first, to determine what settings and compensation work best.
An alternative is Fuji Provia 400X, though I think on comparison it is a more greenish toned film under push conditions. This can be good for builidings and automotive, though not quite as nice on skin tones.
Ciao!
Gordon Moat
A G Studio