Eh? Its not that simple.
This is only true if A) The subway is shallow (I have a feeling Brampton won't be), and B) The line is fully grade separated for a long stretch of the line. While the former might still be somewhat true (hopefully its not the TYSSE), the latter isn't true. The best way I can demonstrate this problem is with the new Central Subway that opened in San Francisco this past year. Its a 3 stop extension of MUNI Metro T line (A mostly median running tramway) into downtown SF via a new tunnel. This section of the line, is no joke, SLOWER THAN THE EXISTING BUS SERVICE THAT PRECEDED IT. Why? Because not only does the line have horrible headways, it is also deep that by the time you could've possibly recouped any time savings from those 2 factors, you're already at the outdoor portion and aren't going to go much faster than local busses.
Now in all likelihood this Hurontario Subway won't be anywhere near as bad as the Central Subway, but fact of the matter is having the last 2 stations of the Hurontario Line be tunneled isn't going to make the line that much more appealing to car owning riders when the other 95% of the line isn't grade separated.