It's nice that you're posing the question within the context of a single line from one manufacturer because then we don't have to get into the whole "relativity" thing that stems from the vagueness of grit size labeling. But your question poses some of the same basic issues.
A grit number on a Japanese synthetic stone is presumably the manufacturer's way of telling you the "screen size" of the abrasive mixed into a soluble abrasive. This should tell you something -- but not everything -- about the speed of the stone (how quickly it moves metal, how quickly it creates a burr), and where it fits in the continuum of stones in your kits.
However, speed is more than just grit size. Other things which make a stone fast (or slow) are the type of substrate (e.g., ceramic or "mud"), how it's fired, how dense the substrate, the type of abrasive, the concentration of abrasive, and so on. Those kinds of things can make one stone better for certain types of metals (very hard, pm stainless, for instance) while not so good for others (hard carbon). By way of one example, the Shapton Pro series has a few stones which are like that.
The good news -- as far as you're concerned, anyway -- is that Naniwa's Chosera series stones are not tailor made for any particular type of alloy -- they're "one size fits all" and very good at it. Not to argue with Franz, but all Chosera stones use the same type of substrate (magnesia), and abrasive (aluminum oxide).
The easiest way to think about stones is the way they fit within your kit; or if you do a LOT of sharpening or have a lot of stones (you have a lot of stones), the way they fit your regimen for your knives -- or a particular knife. That means using the classification system you want to reject as uninformative with some additional information and nuance.
You already have the basic idea: Coarse; medium coarse; medium; medium fine; fine; extremely fine; and ultra-fine. Coarse stones are used to move a lot of metal in order to profile and/or repair, extremely and ultra-fine stones are used to move very little and are intended only to polish out the scratch marks (aka "scratch," or "scuff") left by sharpening with coarser stones, or -- when "touching up" by use and/or steeling. The medium/coarse through fine stones are associated with making an edge sharper. I.e., the apex where the edge bevels meet becomes both narrower AND finer from stone to stone.
Cutting speed, which is the propensity of the stone to move a lot of metal, is very important. Very fast, coarse stones are dangerous. Small mistakes in technique can have large negative consequences; and those consequences aren't easily repaired on finer stones -- which places a lot of onus on technique. However, an ideal stone is very fast and speed is one of the qualities you look for when choosing stones.
Scratch is as big a thing as speed. Scratches on the edge bevels reach the edge as serration. The ideal edge has no serration at all; although as a practical matter (a) a little serration is inevitable; and (b) for some tasks -- cutting raw red meat, e.g., a degree of serration (aka "tooth") is helpful.
Obviously, everything else being equal, coarser stones are faster than finer stones and leave deeper scratches than finer stones; and just as obviously finer stones are slower then coarser stones, and polish out the scratches left by those coarser stones and/or by use.
A good way to think about how two stones of similar grit level compare to one another is in terms of their respective "reach." That is, how far back will a stone reach to polish out the scuff left by the stone you chose as its predecessor, and how polished a surface will it leave for the next stone or for a particular use.
And now we can get to the difference between the Chosera 800 and 1000. The 800 is slightly faster than a 1000, but not significantly faster than a 1000 when it comes to ordinary first stage sharpening. But the speed makes a difference when you progress from coarse stones. An 800 will reach farther back than a 1000, but not as far forward. So... if you want a medium/coarse stone that reaches down to something VERY coarse, you'd choose an 800. While the 800 reaches the 3000 well, the 5000 isn't fast enough to polish out 800 scratch without a lot of work.
So, if you're using a 3000 as a jumping off stone to something finer, like an 8000; or as the final stone for a knife which takes a lot of abuse or wants some tooth, an 800 is a good preliminary. On the other hand, if your next stone is a 5000 the 1000 is a better choice.
You'd only "need" both Choseras if you repaired a lot of badly used knives with very coarse stones; sharpened a LOT of knives, each according to some well-chosen finish; and were very concerned about using the minimum number of strokes and expending the least amount of time. In the case of the EP, the 1000 is a better choice than the 800; and because of its better choice is a more versatile medium/coarse bench stone.
The same is true as you move up the ladder of grit progressions. If the stones you buy for any given rung of the ladder are reasonably fast and have reasonable reach you'll be fine taking reasonably large steps. That is, you shouldn't need more than three or four surfaces to take a knife from chipped and needing thinning to a fine, polished edge with good geometry.
In other words, the complete Chosera set that CKtG sells as part of its EP Chosera kit and "Full Monty" consists of a bunch of stones you don't really need. Play with them anyway.
Hope this helps,