Platter upgrade for VPI MkIV?

FYI VPI uses a non-inverted bearing as the main bearing in its best belt driven TT, the Avenger Reference with the magnetic platter.

For clarity, the magnetic drive platter uses a traditional non-inverted oil bath well bearing to support the drive platter which also has an inverted bearing to support and isolate the slave platter. 

.....and the major advantage of an inverted bearing is that the majority of the mass is below the bearing / thrust pad's center of gravity. 

I do however, agree with your statement that both inverted and non inverted bearings have advantages and disadvantage and what matters most is the quality of the bearing.

BTW, VPI never made platters with a combination of acrylic / lead or metal, they used Delrin / Acetal.  VPI did for a short period of time made platters out of  both 100%  (frosted) acrylic and carbon infused (Black Knight) 100% acrylic, which by all accounts (including me) were the poorest performing. 

The stainless steel TNT Mk5 bearing with the rulon bushing and heavy stainless steel retaining donut is as good as it gets.  If you have not done already, I would suggest replacing the 0.25" ball bearing with a ceramic silicon nitride (Si3N4) grade 3 ball bearing.  Also, if you have an extra thrust pad, have a local machine shop drill out the tungsten pad and replace it with a thrust pad made from PEEK.