I did use the Kata for long walks and the sling shot trap started becoming uncomfortable after about half an hour or so (about the same time that it took the Slingshot to be uncomfortable). But the advantage with the Kata is that you just switch on to backpack mode and it is really really comfortable. I now use it mostly on back pack mode but put it to sling shot when I need to keep changing lenses. Only issue is that the strap length has to be changed every time you change modes - ie: the single strap has to be longer when in slingshot mode and shorter in back pack mode - it it not just a matter of changing the strap position but also need to change the length which is a bit of annoying thing but not too bad
Have you considered wearing the bag in a backpack still but criss-crossing the straps? If I'm not mistaken, I have read that will allow you the comfort of the backpack config. with the ability to simply unlatch one buckle and swing the bag around in the sling style. Have you tried that? I would be interested to know if it actually works.