Vintage DD turntables. Are we living dangerously?

Gentlepeople.
I'm with Lewm.
"What is missing is an objective way of assessing platter performance that is truly meaningful"

There are so many different approaches out there and a bunch of them perform brilliantly, yet often they contradict each other in design philosophy. One would think that designers would gravitate towards a common theme if there was one path towards the holy grail..... It seems not.

On the topic of adding weight above the manufacturers original design. The thought here that increasing (or decreasing) the moment of inertia, "I",too much would upset
the time constants and gain profile in the feedback loop. I agree that it should be considered. I took great care when discarding the original SP10 MK3 platter and replacing it with my own design, to keep "I" close to the original.

The LO7D is a special case since the outer ring clamp greatly increases the platters moment due to the concentration of its mass towards the circumference. It therefore makes sense to offer a switchable setting for this.

On the other hand Artisan Fidelity offer a, presumably heavy, copper matt for their SP10's. Albert Porter uses a Stainless Steel matt on his simply stunning MK3 rig. A customer of mine, Pass Labs, makes a stainless steel platter to replace the original SP10 MK2. From photos I have seen it is likely that "I" has been increased.

With the SP10 range, at least, there seems to be a high tolerance for different platter moments.

Lead seems like such a good material for audio, right? Not too hard, not too soft. Yet lead is one of the worst ideas ever foisted upon well meaning, naive audiophiles. Even in small amounts it screws up the bass response and lower midrange. Gag me with a spoon.

Regarding the inertia of the platter & servo mechanism, Yamaha GT2000's original platter is ~5.5kg, but Yamaha offered as options a vacuum platter mat (~2.2kg) and a gunmetal platter (~18kg). So if you used the vacuum mat with the gunmetal platter, you'd be at roughly four times the original platter's weight. The shape of the gunmetal platter is the same as the original aluminum platter's shape, so presumably the inertia to weight ratio would be about the same too.

So at least in this case, the manufacturer did not consider a very wide range of weights to be a problem.

"What are the measurable physical properties of lead that could support the contention that it is superior....."

Weight, density, damping coefficient? We're talking about the transmission of vibrations through a material(s), and this stuff isn't exactly unknown.

If you affect the torque to weight ratio of an existing motor system, then you might screw up performance? That rates a duh.

Lead doesn't hold shape and is normally used between pieces of more rigid material. It's extremely affective at killing vibrations. If the idea is to match the record impedance with the material directly under it, then lead is a good choice as sandwich material.
Perhaps Trans Fi type spikes is a better choice? I think Naim Audio has a similar scheme. I've never used them.

Fleib - both the Final designer and Goldmund have clearly stated their design goals. In the Final design their goal is to dump excess energy from the stylus/record interaction to ground as fast as possible. They eschew any form of soft materials - rubber, plastics etc. Kenwood had similar goals with their L07D.
Goldmund have a similar design goal of dumping energy to ground as expoused in their publications on their mechanical grounding systems. If I recall correctly Goldmund's use of lead in the platter was more about adding flywheel mass than the damping properties.
As far as the Transfi goes, I have listened to it at length on both an SP10 & Garrard 401 ( with $15k of arm& cartridge ) and in both instances it was vastly inferior to either a copper ( TT weights ) or stainless ( L07D ) mat, missing big chunks of information and smearing notes.
Unfortunately with mats most folk use them like cables - as bandaids for system deficiencies rather than try to work out the best solution.