Full Disclosure: I fish the Arkansas River, which is fairly high gradient, and ranges from 30' to 100' wide, with everything from cascades to half mile long structureless pools. Significant wind is typical. I typically land fat 14" rainbows and browns, and occasional 16" and 18", as well as a lot of skinny 14s and smaller. In the warmer weather I fish 80% beadheads and 20% kebari, and in the winter Heavy beadheads and body weighted nymphs. I've been fishing tenkara about 2 years, and before that, not at all. I think it is fair to say that I know just enough to be dangerous.
For big fish in fast water my two relevant rods would be a Daiwa Kiyose 33SF and a TenkaraUSA Amago. I've fished about 300 days with the Amago, which you know well. For me it handles big fish OK, but it seems to lack sufficient stiffness throughout to provide good control. I have fished it fearlessly, and it has taken the abuse, but I often felt I wasn't really in control of the fish, even though I was able land them. The Amago is too soft to handle a couple of heavy winter nymphs. Casting turns into a slow lob a best. (So use this description as a gauge of my perspective.)
I've fished the Kiyose about 40 days and really enjoyed it. At 10'8" it was a little short for my style of fishing, but is stiff enough to handle fat rainbows in quick water. And I have to say that the shorter length returns the leverage advantage to the angler versus the 13'6" of the Amago. I just need to improve line management so I become more effective at landing a fish on a 20' rig. With a #4 Level Line it handles the wind well.
The Kozuka 39NT is stiffer all around. Casts tandem heavies (1.5 grams total) fairly well. Casts a traditional #12 kebari on a #3.5 just fine. Where the Kiyose has some suppleness the Kozuka has very little. It is stiff and it feels stiff. But there is no question about who is in control when a big fish is on. As a friend said "You could herd small animals with this!" I have hauled big fish across fast water and into an eddy with ease (on 4X tippet). When I want to steer a fish I can steer. It is night and day over the Amago. (I have about 30 days on the Kozuka.)
On the feel, fit, and finish side of things I like the Kiyose over the Kozuka. It's a personal thing, but I just like the way it feels in my hand. (Maybe it's the shorter length.) It is a struggle to get the Kozuka to collapse at the end of the day. Give Kiyose a small tap and it releases. With the Kozuka it takes several stiff blows for each section. And on a Highly personal note, I found the color scheme on the Kozuka atrocious. It is now a uniform flat black.
I have been poking around trying to find a Kiyose 39, but to no avail. I like the Kiyose well enough that I will probably give it another shot, despite the 2' difference in length.
I have heard that a number of folks prefer the Kozuka over the Kiyose. You probably can't go wrong.