Hi. Re: "...I should also mention that I'm putting in a Detroit TrueTrac posi within the next few weeks...."

Are you really sure you want that? People are always raving about limited slip and locking diffs, and while they're great for the track, or even doing stunts on the street, in really slippery conditions (e.g., ice) there is a noteworthy downside!

I know that you know a lot about traction in snow, because of your wise preference for tall, skinny tires -- it's the right choice in snow, a "secret" that a lot of folks down here in the U.S. really don't appreciate. I use 185-14's (without the aspect ratio, a tire defaults to a .81 aspect ratio), and have happily used 175-14's in the past. I wouldn't go to any 65 or 60 series in the winter.

But also consider this:

When tires spin, they loose what I'll call lateral traction -- the ability to slide sideways. Now let's say you're trying to go up a mild hill in your RWD Volvo, and you've applied a little too much throttle ... oops,....
First consider the usual open differential: the rear tire with the least traction will start spinning, of course, but the other tire doesn't spin and holds the rear of the car in place on the road. You may not get up the hill (unless you've got good studded tires, like my studded, Nokian Hakka-1's), but your car isn't fishtailing wildly either, and is staying straight ahead.
Next, consider your intended locked differential: both rear tires start spinning, and since neither tire exerts lateral traction, the rear of your car is fishtailing, and your steering to control wild oscillations (side to side) from winding up spinning you around.

Give this some thought. And in any case, enjoy the winter.
Ken