Chad,
I personally haven't, but if you've got some good profiling data from WMF usage of this stuff it would certainly be helpful :)
Well, in simple micro-benchmarking, wfMsg("pagetitle") took 0.5ms on enwiki and 5ms on frwiki (thats _without_ any initialization overhead). First call to enwiki's message (initialization of message cache, etc) took ~4ms, on frwiki - 14ms.
Tight runs of 1000 wfMsg("pagetitle") calls ended up running for 5s on frwiki and 0.5s on enwiki.
So, now that I somewhat debunked that it is initialization-cost-only, let's move on to actual work that is being done.
For your comparison, I made two profiles for 10-count-run (unfortunately, xdebug profile would take way too much space, if I attempted to do longer run), one for optimized, no-{{ execution path: http://noc.wikimedia.org/~midom/messages-codepath/en.png
And 10x-more-expensive one: http://noc.wikimedia.org/~midom/messages-codepath/fr.png
So, apparently, if we want dynamic messages, we have to spend not just 'additional cycles' ;-)
It is much easier to read those graphs if you take a look at call counts (anything what is 10x or more is usually being executed for each wfMsg() call, whereas other stuff is initialization-only). Also do note, as I was running it in command line, AutoLoader is way higher than it should be, so setup costs are quite inflated.
Also, the base is 55%, remaining part is mediawiki framework setup (configuration, databases, etc).
Anyone, one can see, lion's share is Parser->preprocess, which doesn't do any initialization here, pure parser magic (though it probably has some revisits to magic words and Title code that could be removed). So yes, parser init adds about 5ms, so does message cache, but then every message that has {{'s adds up 5ms each - but message cache init will be much cheaper once new cdb-based code goes live.
Anyway, if anyone still thinks that individual interface messages on every page should take 5ms each to render, let me know, I can unsubscribe you from this list myself, you won't have to worry about that.
Cheers, Domas
P.S. If anyone wants to look at raw profile files, they're at http://noc.wikimedia.org/~midom/messages-codepath/