Tdodd, very well written, well stated, and while it may be a cliche, your pictures speak a thousand words.
The most telling part, however, of your post is "take the strain off my back and shoulders." You must be very strong, and I say that seriously. That is a lot of weight to put in a slingbag. I find myself constantly moving the strap for comfort on the 200 if loaded up.
Yes, the 300 is large, no doubt about it, but as far as ungainly, it's on the back so not really visible to the carrier. I've walked the crowded streets of Manhattan with it hundreds of times with no problem whatsoever.
I hear ya about backpacks and placing down on the ground, whether muddy or not. I don't like that.
As to belt systems, I have two of them, including the older version of the think tank, and I find them to be very cumbersome when walking around in the city. For a hike or even a wedding they are perfect, but to walk around with huge bulges off one's hips in the city is ungainly.