The numbers Metrolinx provided suggest that the Richmond Hill Line will be faster than Line 1 if you're taking it to destinations South of Queen Street, meanwhile destinations north of that will be faster with just Line 1. Queen Station itself is a tie. I have written several long essays on this thread about all of the problems with "replacing YNSE with GO improvements" and I frankly don't want to do that again, but in short, it will help divert some traffic, but its not a silver bullet.
However in the context of Richmond Hill Line being used to supplement Line 1 and the YNSE, really it depends on A) Getting access to the North Toronto Sub, and B) Reconverting the Don Mills Trail back to Rail (good luck with that). Get both of those things done and you're free to electrify it and turn the line to something that could be pretty useful as an express service between RHC and Union.