Yes. LRT makes all stops. No express service is (reasonably) possible. 12 minutes CC to Renforth is possible because it makes only one intermediate stop.
According to distance measurements on Google Maps, the distance between City Centre and Renforth is 9.88 km. The schedule for GO Transit's route 29 indicates that at the peak of rush hour, heading eastbound, a bus will leave CC at 7:55 and arrive at Renforth at 8:07, a travel time of 12 minutes, as you said, with an average speed of 49.4 km/h.
Schedule source:
The schedule for MiWay's route 109, an express route, but one which makes all stops along the Transitway, shows on Triplinx that at a similar time, a bus will leave CC at 8:00 and arrive at Renforth at 8:15, a travel time of 15 minutes, with an average speed of 39.52 km/h.
Schedule source:
Triplinx is the official trip planner and transportation information resource for the Greater Toronto and Hamilton Area.
www.triplinx.ca
A few things stick out to me from this analysis, the first of which is that if a bus making all stops along the Transitway achieves an average speed of 39.52 km/h, why would the LRT run at only 30 km/h? Keeping in mind that this information is sourced directly from the GO schedules, they have to take into account passenger loading times, and the reality of the services as exist presently is that everyone can only board through one door, which considerably slows down travel times compared to an LRT vehicle which can have 4-5 doors and in virtually every circumstance (I have yet to find an exception), utilizes all door boarding. This makes the LRT extremely time competitive to the 109 and given that the 29 uses predominantly double-decker buses, which take longer to offload than a single deck MCI coach, the times should come decently close to the express bus timings as well.
The other thing I notice is that the difference between 12 and 15 minutes is extremely trivial.
LRT has its place. But it isn't always better than buses just because it is more expensive. And putting LRT in places that are inappropriate (Line 5 is a wildly inappropriate use for LRT) locks us into a compromise transit solution that is more expensive or less effective than other alternatives. The idea that LRT will ever be upgraded to higher order rapid transit is quite dubious. You can accomplish a lot with BRT, and by the time BRT is reaching capacity you might be better off skipping over half-pregnant solutions and going to a real rapid transit alternative for only a slightly larger investment.
So, under what circumstances, precisely, would an LRT be the optimal choice, in your book? The statement "You can accomplish a lot with BRT, and by the time BRT is reaching capacity you might be better off skipping over half-pregnant solutions and going to a real rapid transit alternative for only a slightly larger investment." does not allow for any such possibility, in so far as I can tell, and your assertion that it would be only a "slightly larger investment" is fairly dubious and could only be true in an insular debate about the underground portion of line 5, which of course this thread is not about (I hope no one would suggest running any possible LRT along the Transitway underground!). Grade separation, be it underground or elevated, adds a huge cost to a project that simply laying rails on level ground cannot come close to.
The advantages of LRT over BRT are significant, you are able to achieve much higher capacity, should it be required (you can't couple buses together, but you can couple trams) with minimal to no upgrades of infrastructure required if you plan your system right, and the comfort of a nice, smooth, quiet LRV over a bus with an extremely loud engine, bashing your brains in as it hits every bump in the road (a very real, and very unpleasant drawback of the present fleet of GO's double decker buses) should not be discounted, either.
