Skip ahead to about 29:20 in SRT's review of GT5. They say the G27 IS FULLY SUPPORTED. Everything works, including all buttons.
Did I miss something?
If I'm right, they also pointed out that break pedal on G27 is too sensitive. But it shouldn't be a deal breaker.
At least, Kazunori-san was interviewed about G25 and G27 official support and he answered it is work in progress.