Strehl is independent of aperture. It's the other way around -- the "adjusted Strehl" that removes the effects of the central obstruction is misleading. One can remove the central obstruction from the Strehl calculation, but pretending the co doesn't exist doesn't eliminate its effects on the image. The 35% obstructed scope still has a reduction of brightness of the airy disc from 84% to 64% with a resulting increase in brightness of the first and subsequent diffraction rings. There should be two terms: "True Strehl" where 1.0 puts 84% of the light in the central airy disc and can be used to compare scopes of any design, and "Adjusted Strehl" that removes the effects of the co and is really only valuable in comparing one scope with another of the same size co.
Since you seem to be contradicting yourself in this paragraph I don't quite know how to respond. As it is usually defined in professional optical engineering texts the Strehl ratio is independent of both aperture and central obstruction. It is intended to tell you how close your design approaches the limit set by your aperture and central obstruction. It is also intended to tell you how close an actual telescope that you have made approaches that limit. When we finally decide on a telescope and go into the store to buy one what we really want to know is does the one I bought live up to its full potential and if not how close does it come? Only the pure, unadulterated Strehl ratio can tell you that.
In essence the Strehl ratio is like a dog show judge. You ought to be able to judge a group of Beagles and determine which one of them is the best but how can you possibly fairly compare a Greyhound to an Australian Shepherd? The only way to do this is to set a standard for each breed and judge each dog on how close it comes to its own breed's standard, which is what the Strehl ratio does for telescope designs. The overall winner is, in theory, the dog that comes closest to its breed's standard. In practice it is all too often one of the three sizes of Poodle. The same thing happens in astronomy, even though the Strehl ratio gives you an objective measure that does not depend even slightly on the preferences of a human judge the conclusion all too often is that the refractor is best, no matter how large or small. And that is because unlike a dog show judge the Strehl ratio is incapable of comparing an SCT to a refractor.
The correct use of the Strehl ratio is to tell you if this SCT is the best SCT it could be, is this Newtonian the best Newtonian it could be, and so on. The question of which dog to buy is another matter entirely. Buying a Beagle because one won the Westminster Dog Show this year is a silly thing to do if you want to herd sheep or win at the local dog track. Yes, central obstruction is always an important factor and yes, image quality is always improved by choosing the smallest one available. The question you have to ask yourself is whether or not concentrating on one factor alone will get you the telescope that will deliver the most value to you? If you are exclusively a planetary observer in an area with excellent seeing then central obstruction should have enormous weight in your telescope purchase decisions. Otherwise there are many, many factors that go into deciding on the telescope that will deliver the most value to you.
I could not agree with you less on the subject of open and globular clusters. If it fits in the FOV any star cluster gets better and better as the aperture increases. M13 looks like a pale ghost of itself in a 150 mm anything. In a C11 and a C14 it looks truly grand. Someday I am going to have to get an 18 inch Obsession. Yeah, my Megrez 110 gives me a nicely framed view of the Double Cluster that I simply cannot get in either of my SCTs. The trouble is that in the small aperture they barely look like clusters, more like two chance groupings of field stars. In either SCT I can see only one cluster at a time or a view that includes part of both. In the SCT they actually look like star clusters however. I buck the trend of SCT owners who are enthralled with their 80 mm or even 66 mm piggyback scopes. In my opinion they give you a wide angle view of nothing because their light grasp is so much less than the main instrument. At 110 mm I am getting barely enough depth to say that the hassle of dealing with the piggyback is paid back by the view through the eyepiece. Relatively few star clusters have members that are bright enough to show diffraction rings at all and are largely composed of dim members that are as pinpoint in appearance as in any refractor. Star clusters look just glorious in a large SCT.
I can remember reading what appeared to be the first discussion of central obstruction in the pages of Sky and Telescope years and years ago. The purpose of the article was not to convince amateur astronomers that central obstructions were the spawn of the devil. Rather it was to point out to Newtonian owners, who have a relatively free choice in the matter, that the vignetting produced by a small secondary was relatively minor compared to the contrast gain that it would provide for planetary work. A pretty modest and sound engineering proposal. From that beginning has grown a movement that lately threatens to become an all out war on central obstructions. The leaders of this movement would have us SCT owners believe we should be like the children of the 1950's who were afraid of the Communists hiding under their beds. Instead, I would say that everyone who wants to make an informed telescope purchase should be aware of the tradeoffs. In return for a larger central obstruction the SCT gives you a telescope that is remarkably compact and can usually be had on a tracking or goto mount for less money than any equivalent telescope. It will not excel at anything, yet it will be able to do everything you ask of it. It is not the ideal telescope for everyone. It might be the ideal telescope for you. If you want to know what a telescope's central obstruction will do to you, study MTF curves. If you want to know what its aperture will do for you correctly scaled MTF curves and standard light grasp equations will tell you that. If you want to know how closely it approaches the performance the MTF promises the Strehl ratio will tell you.