Well I've ruled out diamond films, very expensive, $43 for a 9x11" sheet, and they don't last long.
You can have the Geshin set, 400, 2K, 6K, for 2 bills. I hear nothing but great things for feedback, finish and cutting speed. Though not soft they will dish faster than your Shaptons, particularly the 400. They also make an 8K, but think about a diamond loaded strop for going higher, 1 micron and 0.5 micron slurry. That will be my next thing to fool around with after I get a current project done.
If you want REAL fast there is the Geshin diamond stone set, 1K 6K, for 3 bills. They dish glacially slow, but it is recomended not to use them against soft stainless jigane as they tend to tear ****** out.