I think variability is inherent to hand built instruments. With more modern more production line oriented manufacture, there's more consistency, but still an element of variability.
We've a DVD which follows Stenway's chief tech in Vienna as he sets up pianos for top artists (Alfred Brendel being one of them). Real eye opener for a number of reasons. Firstly the core one of each individual concert grand, secondly the options available to him to get different sounds out of the piano, and thirdly how much time it takes to get it set up just right for the hall in which it's going to be played. It comes over just how much variability there is between instruments, even at this level, and just how demanding the artists are. Their best example had been sold and the search to find another with such qualities was long and ultimately unsuccsessful - there was a makic something about that one piano.
Now I see Ben searching. Must admit when I first saw the thread title I thought "Here's another guy just after a MkVI for the name." Sorry Ben, no offence meant. I've read a lot of posts where excellent players say there's a lot of variability in MkVIs. And that most are good, but not that good. Add in the years of hard use and many examples are not that good at all. I've never played one. The ones I've heard haven't done anything for me. And my first teacher used his examples every now and again in lessons. But he sounded better on other saxes (King Zephyr alto, and a post war Keilwerth Toneking tenor). I wasn't able to compare his tone on bari as he only had the MkVI. Keywork/feel was lovely, but that was as far as I was allowed to go...
But as I read more and you explained more I realise that like others, you've got a very clear goal and all the right reasons for wanting a MkVI. Reminds me a little of what Pete mentioned on the forum about his search for a new tenor, he tried many, but nothing had waht he was looking for - until he tried the Two Voices R&C, and then he had to have it. I wish more people were like that and not just chasing them for investment value or bragging power. Good luck in your search!