With the G27 you do not have access to the horn or wiper button. You can not map it to work either, you also have no access to the settings on the fly like you did with the DFGT's little red wheel.
Congrats on the purchase you will love it. I love mine
Damn, i think there is going to be an update at sometime to accomodate the g25/g27 i dont know how true this is its just what ive heard. As for the wheel its perfect in every way well worth the money
For the horn, the only way i found for now is to keep a controller beside me (down on D-Pad being the horn). Not very pratical, but better than nothing !