I haven't heard anything about 6 tracks between Mimico & Union just yet although the corridor is wide enough for it. The only mention so far has been of a 5th track in the Exhibition station area.
There are many obstacles however. The Dufferin, Dunn, Jameson & Dowling overpasses would all need be be demolished and completely re-constructed as there is only space for 4 tracks underneath each currently. Then the bridges over Parkside(2 more tracks), Colborne Lodge(2 more tracks), Ellis(1 more track), The Kingsway(2 more tracks), the Humber river(2 more tracks), Park Lawn(1 more track) & Mimico creek(2 more tracks) all need to be widened.
I do hope it will be a future project. As mentioned, 6 tracks will give GO & VIA a dedicated track for their equipment trains, something that will be needed to avoid the huge bottleneck that area would otherwise become with expanded service.
But needless to say there's a lot of work to be done.