Running WP.com, I'm very sensitive too the performance implications of our stack. A small percentage change in performance could mean the difference of racks of servers for us. At Automattic we've also had a peek behind the curtain of pretty much every high-scale WP installation out there.

The vast majority of performance issues I've seen are not from something intrinsic to PHP or MySQL, but how people configure them. (MySQL is especially confounding.) There have been ports to other DBs but nothing that matches the performance of regular WP with a single layer of caching, like Batcache or Supercache.

For dynamic pageviews, ck2 is correct, our biggest barrier is just the amount of code we're parsing and executing on every pageload, which a different DB backend would not change. This problem is bigger on WP.com. I think we can fix this by becoming more conservative and clever about what we load for a given page and it's something I expect to be addressed bit by bit over the next few releases. (BTW ck2 if you want to work on that, please ping me!)