+1 on the Retrospective and the comfort of the strap. I have a Retrospective 7.
I also have a Kata 3n1 that I used with a 50d, 10-22, 15-85 and 70-200 f/4IS. I was able to use it as the bag was intended to be used by its design. Frankly, however, the shoulder straps were just no that comfotable for me and when I moved up to FF with heavier gear, I did not consider getting a larger one. The problem with all sling bags is comfort (I also used a Lowepro Singshot with my Rebel many years ago). If the Fastpack is not working for you, I don't think any sling type of bag would either.
Just today, I had my 70-200 f/2.8IS, my Speedlight and my EF-100L 2.8 Macro in the Retrospective 7 with my 5diii and the 24-105 in a Crumpler 3 million with the straps crossed in front of me (like bandoliers). This setup worked, though it was kind of awkward. I really looked at a lot of backpacks, but still have not found a better solution.