I've never flown YWG-YYZ, but it seems to me from all I've heard that those flights are usually very full. Maybe it was the season, but the couple of times I was at the airport this summer, the YYZ flights were oversold, and I know of some folks who couldn't get tickets to YYZ. I think an Onex merger would mean some 767's on that route.
I think a better example of duplication of routes that would consolidate aircraft would be the YEG-YYC shuttle route. With the two airlines they fly this route with a variety of aircraft, but many times they are just Dash-8's or F-28's, even though it is one of the busiest routes in the country. In the short term more DC-9 or 737 operations with high-frequency would make sense, with either a large RJ (ie-BRJ-90X) or a 717 or A318 would be good for this route in the future.
To add to the DC-10 discussion, CP is selling DC-10's (owned) and leasing two 767's. Also, the mentioned A320 orders were deferred for years now, scheduled for around 2002 right now, I think. The single A320 that CP acquired this year was a used aircraft on lease.
By the way, the National Post newspaper today said that AC may be coming out with an offer to buy CP as early as tomorrow. We'll have to watch and see, if so it would probably thwart the Onex offer for AC and CP.