The @w3cWorld Wide Web Consortium WebFonts working group now has a fully functional Demo of Incremental Font Transfer which compares IFT to normal font loading with Unicode-range static subsetting. Just click on "next text sample" to see the font being upgraded in real time to support more writing systems and more font variation axes. There is a running total of bytes transferred, you can see that IFT loads much less data.
Both CFF2 and glyf fonts are used in the demo.