There should be three additional stops - Woodbine Racetrack (connect with VIA/GO trains with a short run to the airport as well as MT, BT and YRT and a quick hop to Humber College) Mount Dennis (connect with the LRT subway at Eglinton) and Parkdale or Liberty Village at Queen or King. If you use modern EMUs or even DMUs, they'll have pretty speedy acceleration times compared with GO bi-level locomotive powered trains. I'd even consider trading Etobicoke North for Woodbine.
Two branches - one to the airport running every 20 minutes (it would take 28-30 minutes) and one to Brampton/Mount Pleasant running every 20 minutes would give you a damned good rapid transit line for northwest Toronto.
Fine (and I have always wanted GO to build more stations on that line including/especially the Liberty Village one) but those will cost money.....the RFP was for an express line to the airport from Pearson. 30 minutes would, probably, only be considered "express" by business travellers at the most extreme times of rush hour...most other times a cab can get them to the airport in 30 minutes.
So, while the thing should not be built exclusively for business travellers, we have to accept that they will be the most common users of a train from Union to Pearson......if you design and price it to discourage them you have just built a new GO service that happens to go through the airport.....nothing wrong with that....it just wasn't the plan that the proponents (and others) responded to.
I would guess, that what you are suggesting is something along the lines of what will happen......with the expanded GO service coming on that line, with the ability to add more trains and express GO runs, it will be possible to add more stations while not stretching regular users commute times to much....what you wil get, though, is GO serving the airport and (IMO) an inability to take the trains into the airport (the detour would destroy the line as an efficient commuting tool) so a transfer to a people mover will be needed...which will discourage people from taking the thing as a means to get to the airport and we will look back and say "why did we build that....no one uses it)".
I continue to see nothing particularly wrong with the original thought process which was, as I understand it:
1. Use public funds to improve the core infrastructure
2. Once the core infrastructure is upgraded, you can increase the speed and regularity of the public transit on the line (as has already been announced by GO and Metrolinx...to varying degrees);
3. Once the core infrastructure is upgraded a private company can invest some of their coin to create an express service to the airport to satisfy the business (ie. non-commuter) needs.
1 and 2 have/are happening.....just not sure why 3 is causing such a kerfuffle.