Eric, my point is that any Series 3 is still a Series 3, with or without the bottom section extended. I've tested this with my various tripods and found that the bottom section adds very little additional vibration.
Now, getting down very low, conducive to sitting directly on the ground, with all sections retracted, or one section just slightly extended to adjust to a comfortable height, does significantly help. It is obvious with tap tests and controlled slow shutter speed image tests.
Confusing the matter, I've also seen counter-intuitive results. For example, in my test of a Feisol 3442 with various sections extended, in most cases the tripod performed signifacantly better with all 4 sections extended than with the 4th section retracted.
I attribute that to specific resonances related to the specific payload I used. With a different camera and lens I would not expect to reproduce those results and I did not take the time to try to pursue that line of inquiry.
Now, if I put a 500/4 on my Series 3, and another 500/4 on a Series 5, and go back and forth doing tap tests, I will see a very significant difference. And I have done exactly that. And I would expect the same general performance difference with different payloads.
I've never handled a Series 4 CF so I can only assume the performance relative to a Series 3 would more or less split the difference. I would very much like to get my hands on one. My older Series 4 alloy does NOT outperform my GT3541LS, in tap tests nor controlled slow shutter speed image tests, which I found bizarre. I could have lost a lot of money on that bet.
I have tap tested my 4 Section GT3541LS against a 3 section GT3540LS, or some 3 section variant of that, both with 500/4's and both with TC14E's. I saw not enough difference to conclude that there would be any significant value in swapping my 4 section for a 3 section. It was not even clear to me if the differences I saw were real or observational error. When it's that close I call it "a draw" and look for a more significant solution.
It is due to the above, and other tests I have actually performed (verses trying to reason this through) that it bothers me to even suggest, in any way, that somehow a Series 3 can "act like a Series 4", in any way, simply by retracting a leg section and adding some tube length.
It is true that, in general, more sections adds instability as well as longer tubes (or a higher working height). But it is not necessarily true in any given situation and even when it is true to form the differences can be quite subtle. It's one reason I do not do as much tripod testing as I used to. And also why I do not see an XLS as some form of *assured* significant stability improvement over my GT3541LS.
The more I test the less confident I am that my results might be applicable in any but the very specific configuration and environment it was tested. The only thing I am sure of is that increasing the Series will absolutely increase stability, and significantly, in a way that is unambiguous.
I think you need to decide if you are losing more images to sharpness issues or more images to height issues. Or, do what Jim Gould (jgould2) did, and get a Series 4 or 5 giant with more than 4 sections, and get a set of feet to remove surplus sections when not needed. With the latest Series 4/5 you *can* have it all now, an "XLS" and a Series 5. If I had a spare $1500 or so that is what I would probably do.