Vintage DD turntables. Are we living dangerously?

Lewm,
You might like to try a copper mat on your SP10 if you haven't already. In a direct comparison of the L07D stainless mat, Boston, and a few others - the copper mat came out as the most complete in my experiments.
A word of caution - for DD turntables mats should ideally weigh the same as original as the speed servos are optimised for the mass of the standard platter. For example in the L07D the manual carries a warning that if you run the platter without the mat the servos will not work properly.

Dover, I totally agree that it is wise to use a mat that approximates the weight of the OEM mat on a DD turntable with servo control. I have preached that gospel for years, but lots of folks ignore the principle and claim to be getting away with it. This is one reason why I stick to the OEM platter sheet on my L07D. That and the fact that the L07D sounds so terrific. I do suppose that there is a copper mat out there that would approximate the weight of the platter sheet (5 lbs, I think). Why do you suppose that copper might sound significantly different from, not to say better than, stainless steel? It's also the case that the L07D servo was deliberately designed to exert a much looser control on the speed than does the Technics servo, for one example. I think it only activates when there is +/-3% speed error.

Lewm -
With the various mats the L07D stainless & Micro 180g Copper were very close on the SP10. It is possible the differences may be more to do with mass & the actual surface profile than anything else, since they are both high impedance type materials.
People forget that platter mats are mechanical devices that not only provide an interface and base for the stylus/record, but also have significant impact on altering the resonant behaviour of the platter itself.
In particular with aluminium platters, a metal mat such as copper/bronze/stainless will dampen the platter much more effectively than say acrylic or rubber. The reason is that if you combine two materials that are close in propagation speed then reflected resonances ( which end up back in the record and stylus) are minimised.
Reflected resonances - whenever a resonance or energy passes through a material junction, most passes through but a percentage will be reflected back. The closer the materials in propagation speed, the less the reflected energy.
This is why Goldmnd and others went the methacrylate way with platters to get the impedance much closer to vinyl and minimise reflections.
In summary the mats on an aluminium platter are trying to bridge the vast difference in the impedance of vinyl versus aluminium. None of them will be perfect.

Yep......

Nope.....
Doesn't add up. The Goldmund mat was about 3mm and close to the mechanical impedance of the record. Vibrations pass through relatively unimpeded and the aluminum (usually) platter would reflect them back to the record. The mat will only take you so far and is eclipsed by the Goldmund platters methacrylate + lead.

A copper or stainless mat will pass the vibrations through to the platter with less resistance, but is dissimilar from the record and is less efficient draining vibrations in the first place.

That is why lead is the material of choice under a mat that tries to match the record. Lead is much more efficient at slowing vibrations and less are reflected back. Given the thickness limitations of a mat it's somewhat of a turkey shoot.