Why will no other turntable beat the EMT 927?

Well, I think Raul points out potential problems with the longer arms and if they are not addressed, then the disadvantages are outweighed by a correctly implemented 9" arm. However, if the effective mass is low enough and the arm and cartridge are properly aligned, then the advantages of the lower tracking distortion of the longer arms can be realized.

Though I'm not sure, I think that longer arms can have the same or lower inertia than shorter arms. It is a function of where the counterweight is located in relation to the pivot. In my specific case, though the effective mass of the V-12 is 1g greater than that of the 9" V, the inertia may be less because the counterweight is considerably closer to the pivot allowing the arm to react quicker.

People who have compared the V to the V-12 have commented that the V is more dynamic and "quicker" sounding. I did not notice this. I did hear a clearer, more detailed and smoother sound with the V-12 though.

Perhaps John Gordon can add some comments about effective mass and inertia.

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.