I think there might be a much easier option in the offing: When the GO bus station is moved south of the tracks, the present one becomes available. Run the westbound cars north on Bay, on the surface, loop into the present GO bus station, unload, load, then south on Yonge and turn east on QQ,
Simple and somewhat cumbersome? Absolutely, it might need priority at some turns, but QQ already has that 'imposition'. Dirt cheap and simple to build? You betcha, and no excuses not to build it soon, and absolutely no reason to close the present QQ West/Union loop to do anything. Those wishing to run through east-west on QQ can transfer at corner of Bay/QQ. Only serious downside would be traffic on Bay north at morning peak, and Yonge south evening peak. So future considerations could be considered to alleviate that.
The beauty of doing a surface loop would be to get the route up and running as soon as possible, and relatively cheaply. Improvements can come later. The present Union GO bus platforms and connections are too valuable to waste. And they're already built.
How much traffic could this handle? Think Dundas West station, except with much more room and superb connections in comparison. A pedestrian tunnel from the present GO Bus Terminal to TTC Union Station would be needed, a very modest challenge compared to all the ideas I've seen previously.