The main crosstown regional bus corridor is actually Highway 407, which is served primarily by GO route 41/47/48. That is GO's busiest bus route, where most of their double deckers are used:
https://assets.metrolinx.com/image/u...25/TABLE41.pdf
There is already a 407 Transitway being planned (proposed?) from Burlington to Pickering with 50+ stations. The 41/47/48 bus does use a small portion of the 401 in Scarborough, but it's mostly about the 407.
If you think about underground bus tunnel, it was needed in Ottawa. They built this grade separated BRT, but downtown, the busiest section of the system, where all routes converge, it was on-street, so it was a major chokepoint. The 401 is not such a major focus for GO, and the 401 is not exactly Albert and Slater.
Look at the GO Bus system map and you can see the 401 is served by multiple routes, but maximum combined frequency along any section is only 15 minutes. The 407 in comparison has 5 minute frequency in some sections.
https://assets.metrolinx.com/image/u...al-bus-map.pdf