OK, you did the calculations but what exactly is your question? If you're thinking of getting new tires then the answer is pretty simple--just stick to the stock sizes and you'll be fine. If you want a baseline or reference point, I think you should use the tire size that came with that particular car in question as your reference point. Like you said, the 850 lineup has a few sets of tires as OE but I think the real question is whether or not Volvo actually bothers to re-calibrate the speedos and tachs based on these different tire sizes they use as OE. You understand what I'm getting at?

Here's my own experience: My car came with 205/50/16s and I've been running with 215/40/17s for about 4 years. This size is smaller than the OE size BUT with the rims that I had on the car and Eibach lowering springs (about 1 to 1.25" drop), even these tires rubbed during hard cornering. I recently snatched up a second hand set of Volvo-factory winter steel rims with all-seasons (sized 185/65/15) and these rub a little under hard cornering as well. So there you go, this is my experience and I'll let you know, there isn't a lot of room for negotation concerning tire sizes on the 850, IF you lower the car. My next logical step would be to dial in more negative camber (to -1 degree) in the front and hope this will cure the rubbing problem I'm having.