If we get to 15-minute off-peak GO service, is that subway-like intervals?
Yes, definitely. I think that's all we need.
15 minutes is not bad. but even with 5 minutes headway, the GO still won't be the same thing.
Because it is not really just about headway. Regional trains in those cities are seamlessly integrated with the subway. For example, in Tokyo or Paris, you can hardly tell if you are taking the subway or the regional trains because they work together, with many interchange stations.
Go and the TTC are still completely different systems and one can immediately tell the difference. There are very few interchange stations and the transfer is not seamless (you get out of one system and get into the other), if I remember it correctly.
Additionally, there are hardly any stops in the central area, so for people who live in old Toronto, they are almost useless. For example, on the Barrie line, the first stop after Union is York University. I myself have never taken Go once because it only goes to limited areas in the suburban residential area. And if you look at the yamanote line in Tokyo, it is essentially a ring line in central Tokyo that people take on a daily basis.
This is why I said the GO isn't the same thing as the regional trains in Tokyo, Berlin or Paris, which serves the suburbs as well as the inner city. The GO serves the suburbs exclusively.