I am not sure how Go transit is going to help with traffic on the area's busiest days around the mall, namely Saturday and Sunday. And Go transit is certainly not a 'rapid transit' option for most of the population in the city or surrounding burbs.
With Landmark, Remington and Splendid coming online during the next 5 years (and yes, most of these developments will take that long to get off the ground) this area requires some major improvements for transportation (including and probably specifically cars). Otherwise, the traffic nighmares of Yorkdale on any weekend (including backups on the 401) are going to look like a quiet Sunday in Midland Ontario, in comparison. And Yorkdale has its own Subway stop.