It is unusual for XP2 Super to be too contrasty. It will generally require about a 1/2 grade more contrast when printing compared to other films. The possibilities are that your film is fine; but the prints are printed poorly. If your prints are made on color paper (a very common procedure), this is quite likely. Try printing the negatives on true black and white paper, and you should get very good results. Judging the negatives without printing is difficult; XP2 Super negatives look different from traditional films.(posted 8082 days ago)OTOH, if the negatives are difficult to print on real black and white paper, (requiring a grade lower than 2), I would say that you need a different lab. (This is of course assuming that you were shooting in a normal situation. If it was extreme constrast, there is probably no other fault).
As for shooting the film at 320, this is fine, and should result in additional shadow detail and finer 'grain'. There should be no modification in the processing.
David Carper ILFORD Technical Service