If you look at the
Lakeshore West train schedule to Hamilton and the
express bus schedule you'll see that travel to and from Hamilton is about 70-75 minutes on the train. Now once they extend full-day services to the James Street, they'll save a bit of time (5 minutes or so), but they'll also lose 10 minutes because presumably most of the trains won't be express trains.
When they introduce all day service to James North, I think they need to also introduce all day express service on the Lakeshore West line. Otherwise the service to Hamilton would be of little use as trips between Hamilton and Toronto would continue to use the faster and more frequent express bus.
For most of the day, the bus is scheduled to take 60 min from Hamilton to Toronto, though admittedly in practice it generally arrives early. In the summer 2013 LSW schedule, weekend express trains (Niagara Falls) were scheduled to take 41 minutes from Burlington to Union, stopping at Oakville, Port Credit and Exhibition. Weekday rush hour trains take 21 minutes from Hamilton to Burlington, of which 15 minutes is from Hamilton to Aldershot. Adding these together would suggest a 62 minute trip, not far off the off-peak bus time.
This is obviously not an accurate estimate of trip time, since the Niagara service uses smaller, faster 8-car trains. But on the other hand, I think the track to James North is better than the track to Hunter Street, and express trains wouldn't need to stop at Exhibition outside of summer months. So overall, I think a 60 minute trip time from Hamilton James to Toronto Union is feasible.
Limited-express service connecting the centres of each city (Hamilton, Burlington, Oakville and Toronto) would make GO an attractive transportation option at all times, not just rush hour. The existing local services provide good coverage, but they are too slow to be very competitive against driving.