There's nothing dodgy about Western Unioning to yourself; people do it all the time. It's not that expensive - less than HK$100 in fees for that much (which is less than a bank transfer). But as you say, carrying US$1600ish isn't that big a deal. My limited experience is that the changers at the airport are pretty good - you might get an extra 0.25/$ if you shop around, but is it really worth much of your time to save 400 pesos?