The transfers you listed are among the worst in the system. Achieving that level of connection doesn't seem like much of an accomplishment. Remember that the longer it takes to transfer, the less likely it is that making the transfer will be the fastest option for people's trip. Even with a tunnel, it would not be a very attractive connection and not that many people would transfer between GO and the subway there.
The effect of building a Main Street pedestrian tunnel is that transferring passengers wouldn't have to go up and down as far, assuming it connects directly from GO platform level to TTC concourse level (the TTC faregates would need to be relocated to the concourse where they originally were). It also saves the time of waiting for the pedestrian signal at Danforth, and reduces the number of pedestrians conflicting with buses turning left on to Danforth, potentially reducing delays. These are tangible benefits, so if the tunnel could be built built affordably it could be a good decision. But if it's going to cost a ton of money, there are probably better places we could be investing in infrastructure.