There are also several new grade separations to be finished after UPX starts operating, that goes all the way to just before Brampton (drum118 mentioned a couple, Torbram and Scarboro). I believe the extra track that hasn't been installed, could be activated by then. All of this will help GO significantly too. Once UPX starts operating, corridor capacity gets consumed, and GO needs to operate "on err of caution" with conservative timetable estimates. Worst comes to worse, the GO trains stay on schedule. Better comes to better, there's frequently longer dwell times and that gives GO room to readjust the timetables to speed up the trains further, especially as extra tracks begin to open, or raised to full-speed operation (temporary speed limit for ballast settling reasons) -- I think in railroading parlance, "TSO" (Temporary Slow Orders).
Some grade separations in Brampton will need to occur eventually (Mill and John St), but that's probably a longer wait due to the difficulty getting things done down that section of corridor.
I suspect it is not too much of a different story than some of the Lakeshore West corridor project which was thought to bring 30min service much sooner, but did not take until 2013 until that was introduced and sub-hour trains to Aldershot -- a few years delay.