Nauticam produces #26308 TTL-Converter. It works with all Nikon cameras, independently of TTL protocol version. That board was specially developed for Nauticam D5/ D500/ D850 and future housings in that line.
Hundreds of people tested it for today. I personally tested it with D850 and D500. It performs very accurate TTL.
Adjustment +/-Ev is available using camera wheel, works for both strobes simultaneously. If you need to set one strobe stronger than other, you have to use also +/-Ev corrector on the concrete strobe body.
Important that #26308 TTL-Converter has Incredible battery life - 5 years.
Device switches from TTL to M mode underwater by camera menu.
In M mode user even can adjust uw strobe intensity manually by the camera wheel, can enjoy holding hands on camera.
I love not having to remove my hands. I remember my Titan housing from L&M and loved that, but the circuit boards were never reliable and I had to spend a bundle to travel with extras. But the idea in and of itself is productive.
I'd agree with Alex as to smaller domes. I normally shoot a 170mm Zen with my D800 and now D850. The last trip I shot at f/9-f/11 or so, which is where I commonly shot my D800, and definitely wish I'd stopped down more. This can be a challenge for lighting though. So larger dome/travel size/aperture considerations are maybe a bit more of a balancing act.
The resolution of the D850 seems to challenge lens/port combos more. One point also is that the setup seems to have been having some front/back focusing issues - more than with the D800. Users are advised to fine tune the focus of their lenses - even FE's with a wide range of DOF. I think my Sigma was slightly front focusing on the body I was using, which was a rental that i received the day before I left.
As to the questions at hand - the Nauticam D850 comes with an optical sync converter. You can upgrade it to a TTL card. The Zen 230 and most domes don't use an extension with the Sigma 15mm FE.
As to challenges - Macro DOF on full-frame is probably the biggest difference I encountered, and I miss using the 60mm Macro and diopters, vs. only the 105mm. Yes, you can shoot the 60, but not really at much magnification.
Thanks, Jack. I do love my 60. Might have to consider a wet diopter. And I'd agree that a focus tune system is essential. I use one and it has made a huge difference is sharpness and since the camera will remember the front/back focus setting when you reattach the lens later the issue is usually permanent. I recalibrate about every year.