Vintage DD turntables. Are we living dangerously?

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.

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.

Regarding additional platter weight affecting performance, there was discussion of this in the past and I remember at least Lew and I participated in that.

The manual for my SP-10 Mk2A states its performance has "stable load characteristics up to 1 kg tracking force". It also suggests the quartz phase-locked system and DC servo motor torque can maintain rated speed even if 500 tonearms of 2 g. tracking force could be positioned simultaneously.

That seems clear to me that a new platter or heavier mat up to 1 kg heavier than the original platter/mat should operate normally.