Electrification will yield improvement to average speed. Using Tokyo's Chuo line as an example, their trains operates at 100 - 130km/h - slower than the Lakeshore lines' speed limit outside of the Union station zone.These types of trips are usually off peak or counter peak and of course these are always a fraction of peak demand. Even if GO is able to absorb all of such potential off peak demand, that will still be far from enough to triple the ridership.
The question is how much of peak demand GO is already handling. Since trains are sort of already frequent on most lines in peak hours, it is possible that GO is already taking care of most of peak demand and untapped demand is not that much. How many more people will get swayed to use GO just because it's 15 min service instead of 30 min. If GO is a convenient/faster choice for them, they are probably already using it. For many others, driving might be much faster and convenient, and change in frequency doesn't do anything for them. It doesn't make the trip faster, not it brings the station closer to their home.
That's why I was suggesting that GO needs to improve its speeds significantly enough that the time saved overrides the inconvenience of not having door to door service or waiting for a bus to take you to GO. The proposed trip times don't seem to be that much faster to me. I am not expecting HSR speeds obviously but is that the best a regional rail can do? How do other regional rails in Europe or Japan with similar average station gap compare with future GO RER?