In regards to coarse stones, I've found that what works best really depends on what sort of knives and steels you're working with.
For carbon/iron clad wide bevels, I find the JNS 300 to be an especially good stone. Doesn't dish too much, has just the right balance of abrasive release to cut without glazing but not become a total mud-fest that'll destroy the shinogi, cuts moderately quickly for its grit, leaves fairly refined scratches for its grit, and is truly 100% splash & go... However, it slows down to nearly the level of a 1K stone if you try using it for chip repairs on stainless or wear resistant steels, or if you're trying to thin stainless mono-steel. Likes a bit of pressure, too, this one.
Shapton GS 320 I find to be one of the best stones I've tried in this sort of grit range for stainless mono-steels, but it's a bit forgettable for me on simple carbons. It also falls into an awkward place of not being as versatile as the SG 500, and not as fast as the SG 220 (Which can still be refined with a fast 1K.). I agree with
@Benuser that it seems to work better on hardened steel, with a smaller contact patch (Edges, or relief bevels just behind the edge.).
The Shapton Pro 320 is a stone I rarely find use for, to be honest. It's slow as all whatsit on anything decently alloyed, and dishes/sheds abrasive like crazy on soft cladding or just simply very soft carbon. However, on simple carbon alloys (10xx, shirogami, etc.) of middle hardness, it cuts very well without the bad habits it normally demonstrates, and is truly splash & go.
For some steels... Diamond plates. Soft, abrasion resistant stainless steels respond really well to them, as do high-carbide tool steels. They also work great on applications where you need to maintain more precise bevel geometry, like on woodworking tools or - sometimes - folders, or fixing geometry on blades that have developed frowns or bird-beaks in the profile from draw-through sharpeners or coarse rods. Not sexy, in the age of vitrified or resin-bonded diamond stones, but more affordable and still effective.
I also sometimes find myself making large jumps in grit from doing heavy lifting on 120-150 grit stones/plates, and having to spend a bit more time on a finer stone, because one of my 500-1K stones is actually cutting that particular steel
faster than my 300-400's.
There's a lot of variables that dictate how these things behave. It pays off to have a collection if you're working on a lot of different steels... Hopefully this helps, though, if your application fits into one of the categories I describe above.
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