Portland Police Bureau motors have run a few sets of the PR3's. One con they relized with the PR3 and siping is that they didn't perform well with the ABS braking system. It is almost like the overgrab counter-acting the pulsating function. Rather than the pulsate they grab and release. This obviously is not what the rider wants out of a bike with ABS.
The PR2's (non siped) dosen't have this issue. PR2 has been a great year round tire. Wet or dry pavement is performs well.
Michelin's website shows videos of wet stopping tests of both PR2s and PR3s on a bike with ABS. You can see the PR3 actually locking and releasing rather than the ABS pulsing. BUT, the PR3 still stops shorter than the PR2. These are straight line tests with outriggers on the bike, so that locking/releasing action might be an issue in a curve.
Michelin also advertises that the PR3 gets 4% better tread wear mileage than the PR2, but I'm finding the price for the PR3 to be well more than 4% higher. So economically, the PR2 is cheaper "per mile" than the PR3 even with the longer wear of the PR3. It all comes down to how much riding you're likely to do in wet conditions. (I do very little.)
Having said all that, I think PR3s are in my very near future. I have 6,150 miles on my OEM Bridgestones and the front is toast. The rear probably still has some life left, but I'll be changing the pair. (Now it's just an issue of 190/50 vs. 190/55 on the rear.)