David, your post pretty much gibes with my experience. As much as I like to think that every one of my natural stones has its place in an ideal progression, I sort of suspect that the belgian blue and the TOS are fairly interchangeable in terms of what they accomplish – really smoothing an edge without fear of overhoning, nicely prepping it for finer hones. I have a big TOS (8x2) so I tend to go to it instead of the blue, and at that size with a slurry, I don't find it any slower.

Also like you I prefer to use something between the DMT 1200 and the blue/TOS. For this purpose I think the Dalmore Blue (another Scotch hone, coarser and a little faster than the Tam) is excellent. The Norton 4K is obviously great for this too but I rather rashly sold mine when I switched to natural hones. Anyway I have the Dalmore in an 8x2; it's actually the reverse side of my Tam. I think these stones are underrated, or maybe it's just that they haven't been tested and endorsed yet by any real honemeister. Simon apparently can get these as well as the TOS.

The one scotch hone I haven't found very useful is the slate-gray "Water of Ayr" stone. It's finer than the Tam, and is specifically for razors, but it's hard to find and for the money I much prefer a coticule.