Based on your User-Agent, Google's CSS service only responds with a CSS to either a .woff, .woff2 .eot, .svg or .ttf font file. For local hosting you'll need to order these formats in a compatibility sequence, so every browser gets the one it supports (nice writeup here: http://css-tricks.com/snippets/css/using-font-face/).
As all files are the exact same ones Google is hosting in the wild, I don't feel like you'll lose any optimizations (which might be the case when using font generation services like FontSquirrel).
That's not correct. I just downloaded the same google font css using different user agents, one for Windows Google Chrome and one for OS X Google Chrome. CSS were different, both specified .woff2 format but actual font URLs were different. And downloaded .woff2 files were different as well. I used Open Sans font for that test. http://pastebin.com/edYbfxzU here's log, you can try it yourself.
I'll look into that and maybe provide some additional fonts on a per OS/Browser basis to download. The main problem is that it's impossible to have a static CSS snippet AND provide different files per User-Agent (a javascript solution might help). So until I've found a solution to this, you are absolutely right, these optimizations will get lost on local use!
For reference: I'm currently using the following User-Agents for fetching the font file urls from Google's CSS: http://pastebin.com/5SZ1Q6kj
I guess you would lose these optimizations by hosting them yourself.