For what it's worth on my '11 SuperSix the stock Tektro brakes worked kind of poorly, even with Koolstop pads, competent (pretty sure!) setup, cleaned rims, etc. Upgrading to 5700 in the front (which matches my drivetrain) resulted in amazingly fantastic brake performance. I think maybe the Tektros weren't really designed for the redesigned cable pull of the 5700 levers? I use my rear brake a lot less and the stock Tektro does fine there--for scrubbing a little speed in turns I was a little over enthusiastic about, and to alternate onto during long descents.
From what I understand, 6700 and 6800 are cross compatible for brakes, and 6800 is meaningfully redesigned for greater braking force. Funny enough, 6800 calipers are actually heavier than 6700 and 5700.
I'd pick between 6700 and 6800 in your case mostly on price and availability. Maybe if you ride in the rain frequently err towards 6800, but 6700 should stop really, really well.