What you may run into is that they won't allow you to park at Poly even with a dining reservation at that time. I have been turned away from the Contemporary even with a legitimate ADR a couple times when things were busy. So not only would you have deal with traffic getting to the Poly, but you would have to deal with the madness of regular MK parking, trams, and TTC, after trying to drive to Poly in the first place, which might make you very late for your ADR.
If it were me, and I had to drive, I would plan on getting started on the road about three hours prior to the ADR, and plan to park in the MK lot in the first place. If you get there early, you can spend time wandering around the Poly, but you would rather be early than late. You can also try posting the question again once we actually get to Christmas week, to get an idea of how bad the traffic actually is in 2016. Some years more people drive than others, and if people decide to use Uber instead of buses, that would create additional congestion since the guest drop off for MK is right next door to Poly.