Get a 300mm or 600mm? Oh the agony...

[mackguyver]

Canon made a deal with Tamron to release the specs. for the Canon lenses, in turn Tamron had to release the first batch of lenses only with Canon mounts.

Where did you hear this? That makes no sense given that they are competitors. 3rd parties release Canon/Nikon mounts first because of sales volume (compared to Sony, for example), not because of a deal with the OEM.