I agree with this sentiment, but I do have concerns...
Let's use a trip from GO Kitchener.
Express: 8 Pre-416 + Mount Dennis, Bloor, Union
Local: 8 Pre-416 + Woodbine-Hwy 27, Mount Dennis, Stockyards, Bloor, King-Liberty and Union
Assuming the UPX remains as its planned, you have 30min headways at Woodbine, Stockyards and King-Liberty. Either way, that simply can't be sustained. And to be fair, we're talking 3 essentially equally spaced stops. I'd have to look into it but, unless you get rid of some RT stations, I really don't know if the difference is a whole 30 mins.
Perhaps this is just a greater symptom of poor network planning. Stations like King-Liberty and Front-Spadina really should be serving most RER Lines, and many of these longer distance trips are probably better served by VIA(though I don't blame Metrolinx for that).