My understanding is that they do not have discretion, however GO has a very active bus operations centre that handles traffic monitoring and informs drivers to make a certain detour if necessary. That's to the best of my knowledge, someone please correct me if that's incorrect.
They seem to really be on the ball--no need for driver discretion when the best they can do is make a best guess, vs. the hard data/traffic maps+cameras that operation has access to. On the Newmarket/Aurora 65 Express down 404 to Union, we would sometimes take the westbound 401 and southbound Leslie/eastbound York Mills/southbound Don Mills back to DVP, and very frequently, almost more often than not, we'd get off the Gardiner well before Bay, sometimes even take Queen's Quay as a detour to Lakeshore. We always seemed to take the best route--if traffic wasn't present or was only slightly problematic we'd stick to the route, if it was terrible we'd go off route and on route exactly where necessary. Good work out of that group.