Updating this thread after taking the 107 the week before last:
"The service also stops at Viscount Station at Toronto Pearson International Airport for passengers who want to board the LINK Train to Terminal 1 or Terminal 3."
Switching from GO to MT was easy enough, and the stop is pretty clearly signposted. Plenty of MT staff about if you needed to ask anything. GO Transit would do well to take notes
The only thing to be aware of is that there's a short walk from the Viscount bus stop to the actual LINK train station - I wouldn't want to do it in winter!
BTW changing at Bramalea isn't something I'd recommend - I tried this on a previous occasion, it's a long enough walk with just carry-on, there is no information or staff at the stop you need to continue to YYZ, and makes for a pretty disorienting and stressy experience if you're not familiar with the locale (like me).
Overall, the Square One transfer to the MT 107 is a far better option, IMO.