I've given it some thought, yes. The pinch point with this proposal is the Lakeshore East corridor and the Stouffville corridor south of Kennedy. You can widen the Stouffville corridor, but the Lakeshore East corridor will need to be very tightly scheduled in order to work.
For the purposes of this, I'm going to break it down between Red Line (SmartTrack) and Green Line (GO RER) services. In reality though, SmartTrack is just the 416 sub-service of GO RER. The way I see it breaking down is this:
During Peak:
-10 min Green Line service from Mt. Joy
-10 min Red Line service from Unionville
-10 min Green Line service from Malvern or Seaton (depending on if the Province wants to chip in to extend it further)
-10 min Red Line service from STC to downtown via the Central Tunnel
-10 min Special Shuttle service from Malvern to Kennedy
Outside of Peak:
-20 min Green Line service from Mt. Joy
-15 min Red Line service from Unionville
-15 min Green Line service from Malvern or Seaton
-15 min Red Line service from STC
The Special Shuttle during peak is key, because it allows Scarborough riders who are either doing intra-Scarborough trips or are connecting to B-D or the ECLRT to have their own train, to avoid crowding downtown-bound trains. It also means fewer trains are required to use the southern Stouffville and the Lakeshore East corridors.