So, after a few direct responses from real estate agents (the guys and gals "on the ground" who expect the 15% tax to have very little effect on foreign buyers), we get this: 

"Royal Bank of Canada economist Robert Hogue said Ontario needed to do something immediately to take heat out of the Toronto market, and the tax was a quick answer, coupled with many other longer-term measures."

Quick answer: no real change to the market. All politics--but perhaps even more understandable than in B.C. where the market had been cooling on its own for 3 months!  Still, election coming up...