Considsidering Replacing a Technics SP 10 Mk II with a MK III. Can I Expect Improvements?

I answered your question on VA.  In sum, the Mk3 is far superior to the Mk2. But as I noted over there, before you compare any vintage DD to any other vintage DD, make sure that both turntables are running up to snuff.  This means making sure that there are no leaky or otherwise defective electrolytic capacitors inside (such capacitors have a finite life span, defined in decades) and that both turntables have been professionally calibrated.  It remains to be seen whether the new 10R is superior to a fully tweaked Mk3; I have an open mind on that.

@lewm
This one is coming from a friend who did the chip replacement for the MN6042 chip. He has gone through the MK III’s he has from top to bottom. Thats why this one isn’t cheap, at least in terms of MK III prices. So yes its all up to spec.

As far as a plinth, I hear no significant differences between my SP 10 MK II is either the factory plinth or the stacked multi density plinth, they both sound quite nice.

For a MK III I would consider finding a B5 plinth, or perhaps doing a cast one from the builder in Germany. Out of curiosity, where have you sourced a slate plinth from?

The 10R may very well change the dynamics of the SP landscape. MK III currently get priced at 60 to 80% of a 10R price tag. If the 10R is better, or even equal, that may very well force MK III prices downwards. Not sure how the market place shakes out once that table gets into distribution. 

It is true that people in 2017-2018 are selling SP-10mkIII more often then ever. The reason is upcoming SP-10r. Price difference from SP-10mkII and SP-10mkIII is very big, at least 6 times. 

I doubt that the sonic difference between a Mk3 and a 10R would be worth arguing about.  However, that makes no difference to the marketplace.  I am sure there is and will be hysteria about the 10R causing a flood of mk3's into the market which will lower the values.  That's good for anyone with a calm disposition.  That said, Chakster, where is the flood of Mk3's; I don't see it.

I had my slate plinth made to my specifications a few years ago.  I bought it from Structural Slate in Bala Cynwyd, PA.  They cut it to my dimensions (23x19x3) and honed it to have flat surfaces on both sides.  I then had the hole for the MK3 cut by RC Waterjet in York, PA.  For that purpose, I gave them a pdf file to program their machine.  I think I got it off the website, Soundfountain, which is devoted to the Mk2 and 3.  I did the rest of the work myself.  I also had a wood base custom made to accommodate my slab.  In the wood base, I installed a large rectangular chunk of brass (from OnLine Metals), which is drilled out and tapped to accommodate a brass rod.  The brass rod stabilizes the bearing housing of the Mk3, a la Albert Porter's idea.  I bolted the wood base to the slate by installing threaded inserts into the slate.  The combination of cherry wood and slate gives some Constrained Layer Damping that seems to help. (I listened to the Mk3 before and after installing the wood base. Sounds better with it.)