We're on ASP.NET MVC4, a stock build.

We've configured route registration a little bit specially* to speed up some common cases and we don't necessarily use every feature in the framework; but we're far from a proper fork.

ASP.NET MVC performs well enough that we get more bang from focusing on our own code basically.

*This amounts to registering our highest traffic routes _first_, and checking left-hand matches before Regex'ing (since we have a ton of "/foo/\d+/bar"-ish routes).

I don't know about Vanilla ASP.NET but they are pretty much using stock config'd IIS and Servers. So I would be very surprised if they weren't using stock ASP.NET (with their own filters, etc using public APIs I'm sure)

Forgive me, I'm not an MS person, but is there no opportunity to augment/customise/replace/mix-and-match/etc the ASP.net components?

From what I've read on http://highscalability.com and the like, they love to tweak.