You can still find some of the old renderings if you Google something like "Silver Spring Purple Line mezzanine". Here's an example (from WMATA): https://www.wmata.com/initiatives/upload/general-plans-silver-spring.pdf (PDF)

When the new contractor took over, it seems like a lot of old materials were yanked from the Purple Line site; I'm guessing some of it may have been the property of the old builder.

I definitely remember the old contractor talking about it; at one of the community meetings a few years back I explicitly remember them talking about the phasing of the construction on the Metro platform for the new mezzanine and entrance. However, since then I haven't really heard anything concrete. I certainly hope it's still in the plan.

Bear in mind that average speed includes station stops. It's really hard to get a good intuition of—it's not like it will tootle along at 16mph; it will run at 30+ mph, stop, run at 30+ mph, stop, etc.

For comparison, I remember seeing (a ways back admittedly) that the average speed of the J buses on East-West Hwy are about 7-8mph. Compared to that, 16mph is a huge improvement. It seems very unlikely that between Bethesda and Silver Spring, for example, that driving would be faster, even with clear roads.