I would argue that to Google, that within reason it is indiscriminate about the speed of page loads. Page loads do matter to ranking somewhat, but readable content will always trump page load speed. That is not to say pages that load slow due to construction techniques are not penalized, they are. But within G's speed consideration metrics, a page isn't unduly penalized because a site with generally similar content loads faster.
I suspect that Google doesn't penalize sites using font-face. Simply because as you say, they are encouraging its use, so I doubt the overhead matters to Google. (Again, within reason.)
@font-face is a good technology. If for no other reason that once it becomes more wide-spread all those sites out there are ripe for make-overs. Which is good for those who do such work. And from my perspective, using font-face does enhance a site. Like with print, I have seen some people go wild because they can. Which is no different than early DTP.
Take care, Mike