IMHO, unless you're doing it for the aesthetics, I wouldn't bother with this if it's only going to be used for GT. There just aren't that many functions to map; I counted 16 (17 if you include Pause). On the G27 you have 6 buttons on the wheel itself, and a total of 12 on the shifter (the D-pad can be mapped to 4 functions). The 3rd red button on the shifter is Start and it can only be be Pause, but that still leaves a total of 17 that you can map how you like. So you could map everything and still leave a button open.
On the PC, however, it's a different story. There are lots of things that can be mapped, as you can have buttons for starter, ignition, calling for pit stops, chat and voting buttons, etc, plus keyboard keys for other stuff.
You don't have a PS button on the G27, so this box could provide that. However, bear in mind that if you want to use anything other than the PS button on another controller, you have to reassign the controller to 1 every time you start the game. The problem is that GT automatically assigns the wheel controller 1 and anything down the line isn't recognized within the game (although you can still use the PS button to access the XMB). I hope GT6 is better about this and allows us to use other controllers not assigned to 1.
If you make this, I would suggest using a wireless controller so it can power on the PS3.