With Bloor in particular, I imagine it will largely self-regulate. If there's capacity, people may transfer at Dundas West to take the train if they value the time saved, especially since for that distance standing room only is a non-issue. If there's no capacity, they will quickly adjust their habits to stick with the subway, as the time savings effective evaporate if you miss just one packed train on a 15 minute headway. While it's possible that they might only use the UPX to travel home, which *could* create a capacity issue for airport-bound customers, I think most people are creatures of habit and will just use the same way to travel both directions.
Mount Dennis is in a similar position, since there will also be an alternative and cheaper higher order transit option there. Weston always seemed to me like the obvious commuter station, since the time savings vs taking the TTC are potentially huge.
I think once RER is implemented, especially if it winds up going to the new Airport Hub, that UPX may be converted to a Union-Pearson direct train, with no intermediate stops in between. IIRC correctly, Heathrow has a somewhat similar connection, where the Heathrow Express runs non-stop between Paddington and Heathrow, whereas the Heathrow Connect (the RER in Toronto's case) makes intermediate stops. Faster trip to Pearson for the Express customers, and RER service levels for the local customers.