http://vancouver.about.com/od/vancou...er-Weather.htm
If you look at this, for a summer tire to perform decent it'd have to be > 7 degrees celsius (45 F) which is effectively Apr to Oct [7 months] of the year.
Given that you already have a set of winters, getting all seasons is like moot because you will be swapping when the weather turns around.
The people who have the as3s down in Seattle don't have a winter set so that's why they got what they got.
In terms of your question about putting equal driver in the twisties, (if the temperature is > 45 F) the PSS will destroy the as3. That's the thing the PSS is meant for - max summer performance. If it's < 45 F, then the AS3 will win.
In any case, if you do go with the as3, u might as well just sell your winter tires as you won't need winter tires in Vancouver weather (unless you go to the mountains to snowboard/ski)