Pete,
It's great that you put a historical perspective on the subject, and your point that it could have proven difficult or nigh impossible to find patching that was consistent in thickness is quite plausable.
I agree that the advantage of quick loading was likely more important than gilt edged accuracy to the frontiersman and that this advantage is lost by using a patch or ball that's too tight.
It stands to reason that the many differing opinions we get on coning are due to variations in the depth, angle,diameter and even occaisional botching of the coning job, affecting the accuracy to different degrees.
As Dan Phariss has mentioned, In John Baird's book "Hawken Rifles" pg. 42 they describe an original barrel with coning only 1/4" long and flaring out only .0025" which seems to be a very light coning treatment indeed, but this may be just enough to compensate somewhat for patch thickness variation without affecting accuracy but, doesn't seem to be enough to lend itself to fast loading.
It seems that to get the fast loading advantage you would in most cases have to wipe out the rifling at the muzzle or come close to it.
If you look at the Hawken & Leman rifle photos of the Jim Gordon collection at least several of the muzzles appear to be coned to the extent that the rifling at the muzzle end appear to indeed be wiped out and a decorative rifling effect filed in at an approx. 45 degree angle to an approximate 1/16" depth or slightly more. Having not seen these rifles personally I can't confirm this. Also unclear to me is whether this fake rifling is typically aligned with the lands or the grooves.
I'm wondering if the original reason for the called decorative muzzle might also have the practical effect of allowing a little bit more lube to stay on the patch during the initial start down the bore.
My interest in this subject stems from wanting my rifle to have a correct historical appearance and still have the best possible accuracy and help others to do the same without destroying the accuracy that they're accustomed to.
Any personal experiences, especially from those who have compared their pre and post coning accuracy on paper will certainly be welcome.