Micro benchmarks sadly have a way of showing nothing much at that end of things. Showing it's no worse/better doesn't even mean it's no worse if the additional/reduced pressure on caches doesn't result in much in the way of additional/reduced cache spills. So you end up doing mirco optimisations that each, taken alone don't do much. But when you get them all together you cross a threshold that has a considerable improvement. I believe SQLite did something like this although probably in a more restrained fashion as they have other goals than being the lowest latency solution against similarly motivated competitors.
Then of course at some point you get a reputation for success so you show the problem and the numbers identifying it and just say "I want to invest X of my time trying Y which I believe has a reasonable chance of helping" And you are told to focus on that and please let them know if you have any more good news. ;-) Sometimes your ideas are genuinely a wash and not worth merging but it is noted that it was worth trying and you're thanked for your efforts.
As somebody else noted in this thread the game has moved beyond much of this kind of thing in some, possibly many instances.
If you're disagreeing, frankly, almost everything you said sounds like a bunch of platitudes... and I don't understand what you disagree with.
If you're agreeing, frankly, almost everything you said sounds like a bunch of platitudes... and I have no idea what your central point is.
Where do you work that allows you to implement everything twice, once in C and once in C++, for a proper comparison to be made?
(I realize that's not quite what you said, but if we're going with 'benchmark', then presumably those places have already done the benchmarks and decided that C++ is generally superior performance-wise.)