I'll bite, but not sure how interesting this info is to others since I suspect most people are quite content with their 259s. However this does give me a chance to thank the authors of this thread since I did find the comparison fascinating and insightful.
As the MEMS folk have said of late, in his schematics Don has indicated some tolerances, and in other areas, he assumes engineering know how. For me, this means that every iteration of a build, I learn something new via trial and error. An example of this are the numerous 2N3565. For example, unless Q3 and 4 are reasonably well matched for VBE, when you modulate the Mod OSC, it will jump a bit at the crossover point. It's audible. My Canadian 259 does that. My other one didn't till I tried some globe top vintage 2N3565s just for fun. I threw them in at random when I noticed the aforementioned. Matching VBE on the globe tops restored smooth modulating. I only noticed because I was looking for it; as I said at the opening line above, in practical use it is probably neither terribly perceptible, neither affecting of the module usability, so few would really care. It is also unknown to what degree the 259s emerging from B&A in the 70s did or didn't have such care lavished upon them during manufacture.
I will say this though - getting the timbre to sound right* is probably the hardest bit. As an example, and without any intention to call out a member to me the videos above by jimfowler, the Timbre does not sound right, especially at lower frequencies where the effect is most perceptible. A scope is helpful here, but only to a degree; the harmonics and the effect of the overtones are best percived by ear. *Here is where it goets tricky, because no two 259s sound alike, so what is 'correct' could be debated. But, and apologies if this sounds cryptic and annoying, what sounds 'wrong' seems easier to perceive.
Here is an example anyone with a 259 adhering to the original schematic can try. For experimental purposes, throw in a 1M trimmer into R210 instead of the stated 5K - adjust as required and listen for the audible difference (actuating the Timbre control) and watch your scope. Sounds different, not bad, just different, and not like a 259. Given the enormous number of resistors in a 259 and the sloppiness of vactrols, this experiment can hint at the 'butterfly effect' in the circuit.
The other thing I will say is that art and the persuit of it should not be perfection and I think that's why I've read articles by e.g. Mark Verbos talking about waveforms that look bad on a scope sounding good. So while I have only the utmost of admiration and respect for Dave Brown, who has helped me with 259 builds, some of the improvements from an engineering perspective can affect 'mojo', or have knock on effects down the line in other parts of the circuit. My other pet peeve is squarish sines. For me it shouldn't just be about a nice rounded peak and trough, but also a 'lazy s' inbetween them, rather than a sharp straight line. Writing about this now, when I have barely touched my system in two months, I can hardly muster the effort to type this, but when I'm knee deep in the schematics and a build, it is the most dear thing in the world to me - the little details, the little secrets. The huge effort that sometimes gets me nowhere, and other times yields different, not better results, but imparts knowledge.