My K100 shoots low as well, both with the original sights and the FO Front and Dynamic Rear I replaced them with. If I found somebody that made an adjustable rear for the K100 slide cut, I don't think I could throw my money at them fast enough. Interestingly though, you can find pictures of old Mk6's that have an LPA style rear in them. I'm thinking its the old Target model before a move to the Elliason style sight.
Another crazy thing is if you get the parts on Eagle's support pages, they have Elliason, Bomar and Novak style adjustable sights listed, but I highly doubt its correct.