I know sendfile originally had some sharp edges, but I'm not sure how sharp it still is? You would need to use sendfile only for plain http or https with kTLS, and maybe that's too complex? Apache lists some issues [1] with sendfile and defaults to off as well; but I don't know how many sites are still serving 2GB+ files on Itanium. :P AFAIK, lighttpd added SSL_sendfile support on by default 3 years ago, and you can turn it off if you want.
I think there's also some complexity with kTLS on implementations of kTLS that limit protocol version and cipher choices, if it's on by choice it makes sense to refuse to operate with cipher selection and kTLS cipher availability that conflict, but if kTLS is on by default, you probably need to use traditional TLS for connections where the client selects a cipher that's not eligible for kTLS. Maybe that's extra code that nobody wants to write; maybe the inconsistency of performance depending on client cipher choice is unacceptable. But it seems like a worthwhile thing to me (but I didn't make a PR, did I?)
[1] https://httpd.apache.org/docs/2.4/mod/core.html#enablesendfi...
If one of the OSs has features that improve performance, why would you not include that in the comparison?
But I also understand, that looking at that might have beyond the scope of the article.
Mind, the whole "its slow get new hardware" comes from the fact that getting another 10% by tuning "won't fix the problem". By the time folks feel the sluggish performance, you're probably not looking for another 10 points. The 10 points matter at scale to lower overall hardware costs. 10% less hardware with a 1000 servers is a different problem with 10% less hardware with just one.
But, still, a tuning blog would be interesting, at least to me.