Is it possible to use the 2008 mk5 GTI Steering controls with the F900BT with an adapter? I haven't really searched too hard on this topic but I couldn't find much about it. Also, how hard is it to install the F900BT in a 2008 mk5 GTI? Anyone have any experience doing it?

Look up the PAC SWI-PS module and see if your car is compatible...if its on the list, yes it can be done.

So all I would need to install my head unit would be the VWAH and the PAC module and i'm done?

VWAH, PESWIPS and a PESWICAN(translates the CAN bus to the PESWIPS)

When you use those, the MFD loses the "radio" tab. You only want to program the 4 left side buttons on the steering wheel. If you program the right side buttons, everytime you use them for MFD it'll do what's programmed for the deck, and when you want to use the deck it'll start changing stuff on the MFD.

As far as I know, there's no way to integrate any deck to a radio tab in the MFD.

VWAH, PESWIPS and a PESWICAN(translates the CAN bus to the PESWIPS)

Or you can get

PAC C2R-VW, PAC SWI-CAN and PAC SWI-PS

Whichever group of harnesses are cheaper for you.