Meanwhile I've gotten great service from the Nanohone 200. It's a bit of a disputed stone, so with experience of the Sigma 240, as well as a couple experiences with the 200 grits range (Pride 220, Pink Brick) and a few varied muddier soakers to cleaner S&G around the 300-400 mark:
For speed with anything you'll throw at it, I've not known better than a Sigma. Scratch pattern will have strays of course but it's very well behaved for speed and wide effeciency. Dishes quite fast, mud unforgivingly effective - not always a con depending on the geometry you're trying to preserve/enhance AND thin, but it also means a wide margin of imprecision and additional work in polishing that isn't going to be clearly levied by the Sigma base speed.
Nanohone's effective speed is not like the Sigma, but it's nothing like 300-400 range neither. It'll be less noticeable perhaps if working on a wide range of steels and geometries because it's in my sense a much more chosey stone, and if I had to generalize, I'd just say it's either not worth spending it, either ineffective spending it, towards grinding cheaper knives, or doing any work of any kind on these. The Sigma was not particularly more... "adapted" to it if you will but it could and would do anything with the expected effectiveness. However most of us deal with rather fancier knives when spending twice the average prices of coarse stones, so I guess it sort of goes with the territory in a logical way. But I do not recommend the Nanohone for an all around at all. What I'd recommend it for is a very quiet scratch pattern with a nice haze already, good speeds, and as precise and tidy as you could wish for in that grit range.
From the various reports about the Debado, I sort of would expect it to be faster than the Sigma, feelingbetter than both Sigma and Nanohone, and be rather tidier than muddy stones, possibly on par with the Nanohone except some more pain with deeper scratches. Which would not be SO much pain compared to some coarse alternatives. And if the Debado 180 is just about what I suppose it'd be, then yeah I totally want to get one.
Last but not least, both Sigma and Nanohone can be effectively dealt with using an Atoma 140. That's perhaps something I wouldn't be entirely sure to expect from the Debado. I admit, again, that because it dishes more, and because it is coarser and releasing abrasives like a maniac, the Sigma pushes the Atoma to its limits much more than the Nanohone ever will. So IF the Debado 180 is just like the Sigma on that matter and no worse, then again, I really have to try one.
My curosity is high with the Debado, and one of my favorite vendors actually has a nice price for it.
Edit... but I've just sold my Sukenari HAP to a relative, and really for the steels in my array now, I don't need anything I don't already have. However being with a relative, the HAP is bound to come back to me, and it needs to be a rigorously continued geometry maintenance at every sharpening, so... it's tempting to try the Debado and have it at hand.