After reading this thread I thought I'd check my holders and found that the number of sheets varied all over the place. I too shoot 8x20 and have about 20 holders, mainly AWB, Korona and a few Wisner holders. The AWB were clearly the best(least number of sheets that could fit) followed by Korona and then the Wisner, Mr W never seemed to have made a good holder though I do have a few of his cameras.
When you consider that the center of the sheet is quite a bit closer than the ends of the sheet when measured from the lens just how accurate do you have to be? There is probably so much slop in LF cameras between the 'actual' focussing plane to where the film finally sits that it probably isn't that much of an issue as you may think.
The depth of the groove on the film back will have no effect on any other holder you are using, whats the problem with having it tweaked to fit your holders?