The way things are looking in terms of development at STC, it will likely see a 10 year gap, if not more, without a single residential building finishing. I believe the last tower finished in 2010 or 2011, and there is currently very little in the development pipeline. A few projects are in the process of getting approved, but residents probably won't move in until 2020 or 2021. hardly a high growth area.
Spadina line is starting to see a lot of growth, finally. A bunch of towers are in various stages of development at Lawrence West, Wilson, and Yorkdale stations. Not NYCC levels, but the line isn't designed for that level of density. Downsview has also seen quite a bit of growth around it along Sheppard since it was opened.
Vaughan centre is supposed to open by the end of the year and is going to have around 2,000 residential units under construction around it when it does.
The Scarborough subway has a much better bang for your buck proposition than Sheppard. Ridership is higher, and the project is cheaper.
North York Centre has been fairly successful, if I had a say in the matter. Scarborough centre, less so.
Sheppard line is indeed very low demand. It just seems busy as the TTC scales service to the demand. The in service capacity of the line is around 11,000 PPHD, less than half of that on the Yonge line. Of that capacity, I would be surprised if 2/3rds of it is even used at the busiest times.