The 407 is free flowing during morning and evening rush hour from QEW all the way to 404. You can easily drive this route, every day, at 120 km/h+ steady.
Sure, not empty. But definitely not "at capacity".
Yes, absolutely.
At least pre-covid however, stretches of the highway were more or less as full as they could be without "going over" and starting to congest, particularly between the 400 and 404.
The 407 is "full" in the sense that it's as full as it can be while retaining the "uncongested" signature it's entire brand is built around. Adding thousands of trucks to it daily would likely tip that balance and force the 407 to adjust it's toll rates to ensure it can continue to be that.
Also, it being as close to congested without actually being congested, means that it is actually effectively at it's maximum capacity. Traffic is known to actually *reduce* vehicle throughput on freeways, not increase.
That said, off peak hours still see significant congestion on the 401 and are times when the 407 has significant amounts of excess capacity. Efforts to lower the off peak tolls on the 407 would be effective in dealing with shoulder-period traffic levels. Perhaps something like "trucks can use the 407 for free outside of 7-9am and 4-7pm"