+1... the difference between nakiri and gyuto (assuming the gyuto is long enough) is small... the difference between those and an usuba is significant. Usuba is much betterI had been practicing katsuramuki with nakiri and gyuto for about a year. I didn't touch my usuba even once during that time partly out of laziness - reaching for what was handiest and what I was already comfortable with - and partly because I figured I was doing alright with my double-beveled knives.
However, when I picked up my usuba just for kicks and katsuramuki'd a daikon one evening, I was pleasantly surprised at how much easier it was to stay thin and consistent all the way through.
There's not much difference, I find, between a nakiri and a gyuto in this role. As some folks say, a nakiri is like a gyuto without the tip and the belly that precedes it. Honestly you could do the same thing with a suji or a petty. It's not so much a matter of the blade size or profile as it is about its geometry. I find that with any double-bevel, you have to concentrate to make sure you keep the sheet thin without breaking through. The usuba, in contrast, just wanted to keep going where it was. I also noticed that I could get move further along with each stroke when using the usuba.
For me, it's a very noticeable difference between the usuba and any of the double-bevels in terms of the ease at which you can katsuramuki and therefore the speed at which you can go. It's also easier to keep the sheet continuous with the usuba, but it's also doable with the double-bevels.