Why will no other turntable beat the EMT 927?

Don't worry about inertia. Just add the cartridge effective mass to the tonearm's effective mass and use the cartridge compliance to calculate the system natural frequency. As long as it is around 10Hz, it's fine.

Tony, I never worry about inertia, except my own.
Peter, I think the total inertia is the sum of the inertia of the tonearm in front of the pivot plus that of the stub and counterweight and anything else behind the pivot. I think what you are referring to is the counter-intuitive fact that the moi of the rear part of the tonearm is related to the square of the distance from the pivot to the center of mass of the counterweight and to only the first power of the mass of the counterweight. Thus, a heavier counterweight that can be moved closer to the pivot will result in lower moi.

Yes, that is my understanding and I think the lower the inertia, the better. So all else being equal, I would prefer adding more weight to the counterweight and sliding it as close to the pivot as possible. Most people don't consider or think about this because their tonearms don't allow the addition of extra or bigger weights. The SME V-12 does, and it improves performance.

So, no, it is not critical, but in my opinion, if it is an available adjustment on a given tonearm, then it should be considered to improve performance.

To my observation, this is one of the design elements that distinguishes the vintage Japanese tonearms from "modern" ones. The vintage tonearms seem to disregard the length of the stub that mounts the counter-wt, and some of them are quite long. Modern tonearms, in general, seem to favor very short stubs and large counter-wts designed to hug the pivot point. Furthermore, the modern designs place the center of mass of the CW in the plane of the LP, whereas the vintage ones typically have the CW higher, in the plane of the arm wand. One exception to this rule is the Durand Telos, which in photos seems to have a very long rear stub in the plane of its arm wand. And its owners are ecstatic over the sound, which just goes to show ya that engineering principles are not the be-all and end-all. Plus, we all know that some of those vintage designs also sound great.

Dear Tbg: No it's not wrong.

We have to think that a higher mass even if matched with the cartridge weight//compliance is always a " problem " because the cartridge /tonearm are in motion so we have to talk here of dynamic mass not static.

We ha ve to think to at stylus tip level at what stylus tip needs, that's it that the tonearm can react fast/immediatly to the stylus tip requirments during grooves riding/tracking and as longer the tonearm as lower the tonearm bearing arm wand response. We have to think at microscopic level and not macro.

In the other side alonger tonearm not only has a longer surface that in motion resonate and vibrate with more dificults to damp it that to shortes surface/arm wands.

A longer tonearm produce additional challenge to the tonearm bearing than a shortes one and a longer tonearm normally suffer on the arm wand stifness/bending that a short one.

In the other side the cartridge signal has to run a longest distance/path in a long tonrarm than in a short one: do you think is not important?, well change the headshell wire/leads in your tonearm and you will have a change in the quality performance of that signal: 3" makes a way difference for the better or worst.

A common problem we almost all have is that we are accustom to very high system distortions/colorations because that's the way we were listening music through the last 30+ years teached by the corrupted AHEE, so when we follow listening those higher distortions/colorations trhouag a long tonearm we like it against what we hear in a short one but IMHO what you like or what I like is not important here what is important is what's right and what is wrong.

IMHO the lower distortions/colorations ( any kind ) in an audio system along accuracy and neutral performance is what really can makes a difference for the better quality performance level.

Now, that that lower distortions and higher accuracy does not like to you or to me is not important and means only that what we like are higher distortions and unaccuracies that are wrong.

Which the differences between different audio systems?: mainly its distortions and accuracy level: not what you or me like to listen because this is totally subjective.

When you listen to live music in a near field what do you listen?: low, distortions, low unnacuracies, neutral sound, natural agresiveness: natural tone colorations and obviously the dynamic power that only the live music can gives.

Regards and enjoy the music,
R.