UP Express trains have a 10 mph (16 km/h) speed limit while passing UP Express platforms so the express trains will hardly save any time at all. They'd be lucky to save 1 minute per skipped stop, in which case the travel time would be 23 minutes.
GO Trains don't have that speed restriction so GO Kitchener express trains commonly pass through stations at 120 km/h, saving about 2 min per stop skipped.
The elephant in the room here is Mount Dennis station which should open relatively soon. Adding that stop to the local service would produce a travel time of about 27 minutes to Pearson. Personally I'm hoping that they just ditch Weston from UP Express and have a all UP trains serve only Union, Bloor, Mt Dennis and Pearson. Removing Weston from all UP trips would allow them to remove the platform edges from the UP platforms, allowing UP trains to pass through at the full 120 km/h track speed. Ridership from Weston is primarily to/from Union, not Pearson, so the Kitchener local service is much more suitable for serving that demand. Especially since UP Express doesn't have free transfers to/from TTC like GO does.