Bob, Perhaps when you get the sample of the Tamron SP 24-70mm Di VC USD Lens for Canon EOS, you could make some comments comparing it to using the Canon EF-S 17-55 on an APC body? As one who uses the Canon lens on a 7d, I would be interested in your assessment of performance change, were I to upgrade to a 5D Mark 3. Thanks, Barry
Unless I can get hold of a Canon 17-55/2.8 at the same time I won't be able to do a side by side comparison since I don't own the 17-55/2.8. However I do have some test shots taken a while ago in conjunction with my review of that lens, so I should be able to make some comments.
I'd imagine that the major factor in the decision to move to a 5D MkIII from a 7D would be reasons of lower noise at higher ISO and a more advanced AF system, together with the general overall improvement in image quality you get from larger pixels and a larger sensor.
Maybe I'll get super lucky and get a 5D MkIII at the same time as the Tamron 24-70/2.8, but given the uncertainties of equipment loans, that would occur more by luck than planning!