I've got your problem as well. Just got a D800. Shooting with a C8 and the same f/6.3 reducer you have I have exactly the same vignetting problems. The 35mm sensor is simply a bit too big. The vignetting is completely uncorrectable.
I've taken the view of more shots is greater and these are my results a few nights ago for the Hamburger galaxy:
https://secure.flickr.com/photos/100...in/photostream
That was something around the order of 80 shots at only 8 seconds each with 20 flats and 20 bias frames. However I was limited by really crap polar alignment at that point. (Guide scope is on order)
The suggestion I got was that the benefits of doing many frames start to drop off above 30-50. Haven't looked into it further yet but I guess that would mean the idea of stacking an infinite amount of frames will not necessarily be the best option and is certainly not preferred to getting better exposure times.
But I'm new at this so take it all with a grain of salt
