UP Express from Mount Dennis to Pearson Aiport would take 14 minutes assuming one intermediate stop (e.g. Woodbine). Driving non-stop, Google Maps estimates 16 to 30 minutes depending on traffic. Realistically a non-stop bus route would need to be scheduled for around 20-40 minutes to account for delays, and a bus stopping at each arterial cross street (along Eglinton or Dixon) would take even longer.
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Until Line 5 is extended to the airport, the UP stop at Mount Dennis will provide a big benefit since it would be the best route to the airport for anyone travelling from North York, Midtown or some parts of Scarborough.
GTS generally increased the track speeds compared to what they were before, except for a 75 mph restriction through the Weston tunnel (previously 80 mph).
According to a post-GTS timetable I found, the GO speed limits heading west from Strachan are:
55 mph (89 km/h) to King Street
75 mph (121 km/h) to Queen Street
80 mph (129 km/h) to West Toronto
70 mph (112 km/h) through West Toronto underpass
80 mph (129 km/h) to Weston
75 mph (121 km/h) through Weston Tunnel
80 mph (129 km/h) to Wice (Pearson Junction)*
25 mph (40 km/h) around the curve onto the Pearson Spur
40 mph (64 km/h) along the Pearson Spur
20 mph (32 km/h) around the last curve before Pearson Station
10 mph (16 km/h) into Pearson Station
* VIA and GO trains are limited to 60 mph (97 km/h) under Hwy 401 due to low clearances, but there is no restriction for UPX.
In earlier GTS planning documents they proposed to increase the track speed to 90 mph from the Weston Tunnel to Wice (Pearson Junction), but that never ended up happening, presumably because it would be pointless with the 60 mph restriction under the 401 affecting Via and GO express trains.
Ideally they'd increase the track speed to 90 mph when they realign tracks into the new 401 tunnel, but I'm not that hopeful given how far the tracks will need to swerve to get into that tunnel. Maybe a consolation prize would be to get 90 mph from the 401 to Malton.