Ok, I have had the VS DNA40 for almost a month now and the VS rDNA40 since Monday.
I like them, they are reliable and just plain feel good. I prefer the rDNA because of the ability to replace a battery and keep on going.
I have used the following toppers...
Atlantis (Kanthal mode) works like any other mod
Kangertech Sub Tank Mini, works great with Ni200 on the RBA section
Kangertech Sub tank Nano, works very well with the OCC coils
Veritas RDA, I did not try a dual coil in this, but single vertical coil works awesome...
Derringer RDA. single coil mode, works amazing, great flavor...
Freakshow RDA, I do not recommend it for Ni200, if you have like 2 hours to kill, it works and taste great, but man what a ridiculously tough build...
In the end, I had loads of trouble with the Sub-tank Mini and the Ni200 OCC coils. On the DNA (little one), they would fire great at first, then they would fire weak and produce little vapor. Then I would turn up the temp as I had heard, and it was great again. Then set it down and do something for a while grab it and fire it and, YOW, hot juice popping up and burning my lips and tongue...
So last night I had some time to kill and decided to do some testing. I have 2 Sub-tank Minis, and two Sub-tank Nanos. I filled every one of them with the same juice and grabbed a fresh OCC coil. I vaped each tank, with the same coil for about 30 minutes each one the rDNA with atty lock NOT on.
I got 4 vastly different experiences, despite the vapor shark reading 0.15 to 0.16 ohms the entire time.
Next I adjusted the rDNA settings for each tank and got a good vape and turned atty lock on, and repeated this for all 4 tanks.
Doing this I got a very consistent vape for a good 30 minutes on each one.
So now I am scratching my head thinking what the heck is going on here. So I started checking the tanks over real good.
I discovered the center pin of the 510 connection is floating and can be moved back and forth slightly in a vertical (up & down) direction.
I found that if I installed the coil and pushed the center pin in from the portion that would screw into the mod, I got a much more consistent vape, on all 4 tanks.
I hope this all makes sense, if not just ask and I will try to explain it better.
But it you have any of these tanks, before you install the coil, push the the center pin of the 510 connection in, install the coil and the center pin should now slightly protrude.
I think the inconsistencies we are seeing with the Kangertech coils is actually the tank.
The reason it isn't as evident with the RBA section, the RBA section is slightly longer and pushes on the center pin a little more... And we don't use the RBA section in the Nano...
I am am picturing a bunch of vapers taking apart their Kangertech Sub-tanks and see this for themselves right about now.

In the end, it is a connection issue as I had thought, but it does not appear to be in the coil, but the center pin of the 510 connection of the tank itself.
I sure hope this helps some folks out...