The DVP between Eglinton and 401 (and the 404 southbound south of Finch) is congested practically all day, 7 days a week. It is not just a rush hour problem. This has got to be the single worst bottleneck on the Toronto highway system, caused because that section of the DVP is only 3 lanes each way wide (except 4 lanes between York Mills and 401) and 2 lanes on the bridge connecting 404 and DVP. In rush hour, the DVP/404 is pretty much gridlocked all the way from Stouffville Road to the Gardiner and it regularly takes 60-90 minutes to drive that section; the DVP/404 combination tends to have the worst travel times of any highway in the city. If you add in the fact that the Yonge line is severely overcrowded south of Finch, this means that a Don Mills subway line to Finch would get extremely high ridership, probably higher than any other proposed transit line in the GTA.
Putting LRT and subway on the same corridor is always a bad idea. The Sheppard LRT was only proposed because the eastern end of the Sheppard subway was cancelled due to Mike Harris cuts, and there was not enough money to extend it. No one would ever propose such an arrangement from scratch, we would choose one technology or the other for the entire corridor. There are always going to be bus-subway transfers in the system, but the difference is that a bus can easily be replaced with a subway extension while an LRT is almost impossible to get rid of (politically) when it gets severely overcrowded.