In all likelihood, your sense of "razor sharp" is getting in the way here, so let's not sweat that. What you want is "so sharp it will do everything I want it to do much more easily than I have any right to expect." Something like that.
At base, the roughly 1000-2000 mark is the core for good-quality Japanese knives. Below this is for fixing things, and above this is for polishing. You don't really get sharper as you go up above this level, but the effects can be a lot like a LOT sharper.
The trick about polishing is that not every knife can reasonably take every level of polishing. You can polish some old piece of junk iron knife to your heart's content, but as soon as you actually use the thing it's all for nothing. So you have to grade your level of polish to the knives you are using. This is why BDL is asking about what knives you use.
Let's suppose, hypothetically, that you are using spectacular Masamoto KS white steel knives, OK? So, how much polish can they take? I dunno -- I don't know anyone who's hit the limit yet. What difference is there between 1000 and 10,000? Well, 10,000 glides like you wouldn't believe -- just drifts vaguely through food in this weird, disconcerting way. And 1000 does not do this -- it acts like (gasp!) a knife.
But if you tried to polish a Wusthof Trident to 10,000, you'd be wasting your effort: even if you managed to get that level of polish on there, which would take some doing, the bending that happens as the edge hits a board or hard food would soon remove all of that. This is not to say that the Wustie is bad, but that it's not designed for this kind of thing.
The other thing is that high-grit stones are not all alike, and some of them are VERY expensive. So before you go dropping huge chunks of change, we'd like to know a lot more about your knives, your sharpening, your cutting habits, and so on. We'd love to advise you -- lord knows BDL and Phaedrus are drooling to talk about fine polish! -- but we don't want to send you haring off after something totally useless to you and expensive for your wallet.