Some nice observations, the exit pupil is an important factor in judging the optical quality of a scope, of the image. "Sharpness" is a subjective judgment, just what does it mean?
I like something more specific, splitting the same close double or showing a particular detail on Jupiter. In my experience, the 8 inch F/6's I have owned did show more detail, split closer doubles than the 6 inch F/8s.
That said, a good 6 inch F/8 has it's place, it can be the right scope for the individual, for the situation. That's what this thread is about.
Like pornography, I find sharpness is difficult to define, but easy to recognize. I am probably using the term imprecisely, but here's an example of how I think of it. When my scope is unfocused, the image is blurry, contrast is low, and I can see few details. When I adjust the focuser to the point were contrast is the highest and the most detail shows in the image, the image is no longer blurry, it's "sharp." We could get into MTFs and such, but I think most layman understand what I am describing and will agree that sharpness is a desirable property in a telescope, and that some telescopes are sharper than others.
However, sharpness a difficult property to measure objectively in the field, and the best I can do is to use both scopes in the field side-by-side over a long period of time, preferably one or two years, assuming that both scopes are of good quality and functioning properly. I consider single-scopes tests a better test of "seeing" tests than a scope test, and don't have much confidence in critical comparisons that aren't done side-by-side.
A properly functioning big telescope will usually split tighter doubles than a properly functioning small telescope because it has greater inherent resolution. If it can't there's usually something wrong with it, so the biggest scope will almost always be the "sharpest" telescope using this test.
Comparing two scopes of identical size on doubles can show which is sharper, but doubles are not convenient targets for comparing two hand driven, alt/az scopes side-by-side at high power, and doubles aren't that interesting to me. I don't see much detail on Jupiter, so don't use it much for these comparisons.
Instead, I like to look at star fields, and I want to see every star as a pin point right to the edge. For this task, I have never found anything better than an NP101. I would call this a very "sharp" scope, but it was one of the smallest scopes I have owned and not as good at splitting tight doubles as the three 6-8" newts that I owned at the time. I also like viewing lunar detail to compare medium to high power views.
Using star fields, I find my 6"f8 a little sharper than the 8"f6 at low power. Primarily because it's smaller exit pupil at the same magnification is a little easier on my astigmatic eyes, and secondly it has a touch less coma at the field edge, but neither scope seems to have an overall sharpness advantage on a wide variety of targets.
Scope selection is very much a personal matter. I want the scope that's best suited to my observing interests, my observing style, and my observing site, not necessarily the "best" scope. I also usually prefer to own just one scope and use it for everything. With that goal in mind, it's usually a 6-8 Dob, depending on the site, but I a tie usually goes to the 6"f8, and one day I hope to own a premium version that lives up to the scopes full potential.