Alex, although I've studied Audyssey technology extensively and now have quite a bit of personal experience in using it, your "how much" question can't be answered with any degree of precision. Nick's post from last night that you're probably referring to said "a lot" and I'd use something like "quite significant".
As to the specific Audyssey equipment, I have no personal experience with the pro kit and certainly not with a pro installer. From my study of the situation I'd suggest neither, but getting the best home XT32 version and following tips such as those in the AVS Audyssey thread on how to make use of its adjustments(incidentally, again, no your equipment doesn't have a "gain adjust").
