Tom actually the advantage starts a lot sooner for primes than 300mm. Once you move away from the Double Gauss design to a true telephoto design is pretty much where the EF-s advantage ends. I'm not sure where the Canon 85mm lenses fall design wise, but 100mm will probably be truly telephoto, with a fast max aperture fixing the size.
The zooms are a bit different, with them it is the wide end of the range that matters not really the long. I am a little surprised that Canon needed to make the 55-250 in an EF-s mount. I have a Sigma 28-300 f/6.3 zoom that is physically about the same size as the Canon, and is still f/5.6 at 250mm IIRC and covers the full 35mm frame. I can understand why Tamron have limited the image circle on lenses like the 16-300, given the necessary size of a 35mm format ultra wide 16mm lens.