TSN released the numbers for the Grey Cup. Average audience was 4.5 million on TSN, peaked at 5.5 million. Surprisingly 2.2 million watched the post-game coverage (I was not among those obviously).
The average was probably hurt by the game being a blowout relatively early. The Cats did close somewhat in the second half and then it was blowout again. Last week's Eastern final was much closer and peaked at 3.7 million, which isn't that far off the Grey Cup peak.