Downsview has same ridership (38.8K) to the current Lawrence East and Scarborough Centre/McCowan stations (38.7K); and that's predicted to increase a bit once there's no change at Kennedy (and no incentive to take bus all the way to Kennedy).
Looking at the
TTC study which compared using the current SRT just to McCowan to a 2-stop subway to Scarborough Centre, there was a 24.7% peak-hour ridership increase just by converting to subway. Assuming this holds for the entire day, then 38.8K would become 48.1K - far higher than Downsview.
That same study also notes explicitly that only half the trains would go past Kennedy at peak - but that during "
off-peak periods, there would probably be no need to change trains (on the basis of TTC minimum frequency standards for subway operation)".
Incidentally, the same study also predicts what would happen to ridership on the existing RT if the Sheppard subway was built from Don Mills to Scarborough Centre. The RT ridership would go up marginally (about 2%). So much for the theory of some that building the Sheppard subway to Scarborough Centre is an alternative to the SRT.