Hello!
apachebench does help with efficiency. It is inefficient (not to mention irresponsible) to wait to benchmark your code until you have a throng of users actually relying on it.
It isn't matter of benchmarking, it is matter of relative resource costs, compared with other projects, and no way you'd get the answer with benchmarking tool. Do note, we have a cluster which serves thousands of other requests, so we may have capacity to serve various exceptions, but we may as well want to spend that capacity on more efficient things, like, um, wikis without eyecandies (interface language changes for english language wikis? way to go!)
Apachebench will allow you to set the header needed to trigger the language content shift in mediawiki while simultaneously hammering the server with such requests.
We already know the costs, we do profile. We have way more detailed costs than anything 'ab' can provide. We have way more capacity in our cluster than a single-threaded benchmark tool can load properly.
Domas