I just want to put some closure to this. I got the ASI6200, EFW, and have it in use with the Celestron OAG. I did a brief trial run last night. I did not get my filters yet (UPS Next day didn't make it).
So here's the first light, it is just 14 x 60s subs with no filter. I was having a lot of guiding issues at first (mostly wind, some tuneup issues), so I had taken a bit more than twice that much but had to throw them away. I'd actually throw all of them away if they weren't really the first light with it.
That's a quick integration and background extraction and preview stretch, no real processing, no noise reduction.
I thought the night was cloudy, but looked outside about 10pm and say clear skies, and grabbed stuff and put it together ... wrong. I had it at 156mm instead of 146.5, about 10mm long. I was pleasantly surprised that despite that I did not see a lot of distortion at the edges. I was also pleasantly surprised that focus looked uniform and I didn't have to fiddle, at least yet, with the tilt adapter.
What I am less happy with is the vignetting. I am not sure if it's the C11 + FR (contributed by the wrong backfocus?) or if the current 48mm link from the OAG to the EFW. I'm kind of thinking at least some is that piece, I need a different sort of connection there.
What did I learn:
You cannot connect the tilt adapter that comes with the camera to the front of the EFW. It looks like it should, but no holes align, and both are thin female threads (in theory I could use the M54 male spacer as a connector but that would be too insecure).
So what I have is:
Camera (Tilt removed for spacing reasons) -> screwed to EFW -> M54-M to M48F adapter to Celestron M48M OAG Adapter.
What I think I want is a M54 -> Gerd CTU -> M54 -> OAG with two precise parts custom made. This will go from 48mm to 54 mm (outside) path and may reduce the vignetting.
Now that I've had some time with it, I'm thinking if I get a small refractor (which I'm trying hard to find) I will actually use the Celestron OAG anyway -- keep the light path the same from it to the camera, and just get a part made for the front. That way I'm using the same basic stuff (and same guide focus distance). The COAG really works nice. Last night on another shot I had only about 3 usable guide stars in frame -- the ZWO OAG has 1/4 the real estate, so I might have had zero with it.