I'm confident that the performance multiple from C# to Ruby is comfortably greater than 2-3x.
But the specifics are heavily dependent on the actual code executed; to a first approximation, most Rails code is shuffling parameters to and from a database. It's very difficult to speculate. If most of your Ruby code, to take an example, is effectively a wrapper around native code (e.g. regular expressions or string substitution in templates), then the win from using something like C# or Java is probably low.
I don't think there's much point nit-picking on multiples - who knows for sure without seeing their code - but my point still stands. Would you have been surprised if StackExchange ran on 50 or 60 front-end servers? I certainly wouldn't have. And that would be the equivalent of being able to take 200ms a response for the same amount of traffic. In ruby/rails that's a pretty horrible end goal. GitHub's mean web response time is currently 57ms, for example.
I would have been surprised to learn it was on 50 or 60 servers. I'm a little bit disappointed to learn that it requires 9, as I had thought of them as one of the last bastions of scale up rather than scale out, but 9 is approaching a large number.
I think the complexity costs of scaling out are underestimated; or rather, they're just accepted as a fact of the matter, like there isn't an alternative. More moving parts in the name of redundancy etc. usually makes things more fragile, without very careful design and implementation.
Interesting that you bring up GitHub. GitHub was probably the most unreliable service our startup used before we moved to Stash. You could count on it being down for a few hours every month, reason given usually "DDoS". We moved to Stash for contractual reasons relating to security and ISO certification required by our target sector, finance, but I could see us having done it for reliability too.
From memory StackOverflow itself only runs on 2 web servers. The rest are for various other stackexchange properties. You'll also note that peak CPU is about 20%, so they could probably run everything on 2 or 3 servers at max CPU (bad!). But they could easily halve the number of webservers and stay performant (but not fault tolerant).
But the specifics are heavily dependent on the actual code executed; to a first approximation, most Rails code is shuffling parameters to and from a database. It's very difficult to speculate. If most of your Ruby code, to take an example, is effectively a wrapper around native code (e.g. regular expressions or string substitution in templates), then the win from using something like C# or Java is probably low.