I actually timed the Downsview Park connection with a stopwatch on opening day. Assuming being onboard the accessibility coach, I got a total of 1 minute and 30 seconds to get from the door of the train to the subway platform. I was walking fast, but not quite speedwalking and not running; unless you run at a dead sprint I don't think 1 minute is possible (and maybe not even then), maybe 1 minute 20 at best. A slow walker might do it in 2 minutes. Elevators seemed slower despite being more direct downwards; the walking transfer would be so much better if the escalators let out on the GO side of ground level rather than requiring you walk all the way around to the other side.
At Union, yeah, 1 minute is laughable. In rush hour unless you line up at the door 5 minutes before arrival, you're at a door that opens right in front of a stairway, and you run at a dead sprint to get into the stairway before anyone else does, then you'll be waiting at least 1 minute if not 2 just to get from the lineup in the train car down to concourse/teamway level. Admittedly not as much of an issue off-peak. I think the best case transfer time, off-peak, for something like platform 5/6, is 2 minutes 30 seconds from the train door to TTC platform level if you're quick and you're at the doors for the bay west teamway or Via concourse. In rush hour, if you're not lined up and you're at the York concourse end of the train on platform 13 (Barrie almost never uses 24-27 anymore, even on arrival), I don't imagine it taking any less than 6 minutes.
Overall I think the typical time savings for transferring at Downsview Park to get to St George will lie in the 0-5 minute range barring any excessive delays on either system. Again, by going down to Union and back up, you're not escaping the possibility of TTC delays, and you're adding an additional very-often-delayed stretch of the GO line. And the $0.98 savings are there, and personally I put a high price on comfort which definitely doesn't describe the transfer experience at Union.
Of course, if you're heading to Yorkdale, Eglinton W, St Clair W, or anything like that it should definitely be faster to transfer at Downsview Park, no question. You can also access stations like Sheppard Yonge (and the rest of the sheppard line), Eglinton, and St Clair via bus/streetcar connections east faster than I think you would get there via Union-Yonge line. That opens up GO commuting to a lot of people who would have driven rather than, say, take the train to Union and the subway up to Yorkdale or Sheppard-Yonge. Can't wait for Lansdowne GO, as well as Caledonia and the Eglinton LRT, to further open up connections on lines 2 and 5.