After six months of use, I think it's worth commenting on the last issue that this long thread discussed: suitability as an octo.
I've been using the D420 on a necklace as my bailout regulator for my rebreather. Doing bailout drills at depth has been an absolute pleasure! The reg purges easily and the case design empties well. The huge smooth air flow is very reassuring should I ever take a CO2 hit and bail out for that reason.
As an octo, I've now done dozens of GIANT strides off the tall bow of a Florida dive boat, easily ten feet from reg height to the water. In predive position, with the reg in a very disadvantageous "mouthpiece up" position dangling from my neck, there's always a brief burst of air as I hit the water. About 1/4 of the time, I'll need to reach up and invert the reg to stop flow. After that, there's never been another peep from the reg.
With only a 40 cu ft bailout bottle for my rec depth CCR dives, trust me - I'd never use a reg that had any tendency to leak gas. That's my life support for up to ten minutes of deco, not to mention getting out of my predicament at depth.
The D420 has been superb as both a primary OC reg, and now as my octo/bailout reg. I now use a pair for sidemount. I continue to be blown away by how nice this reg breathes in the water even if at 1.1" on dry land it doesn't seem that great.