BREDn07, I have an 06 GT. I also have a set of bi-xenon HID lights, also 8000K. I have not installed them yet on the Mustang but I have a set on my F-150 HD truck. There is a very large relay with a direct battery connection supplying the voltage to the two ballasts with a 20A fuse on each. It only uses a connection to the bulb socket for a signal that the lights are on and if the hi beams are on. The guys at Ford did bot know the specifics of my setup but told me that the light controller would freak out with HID's. The setup only hooks up to one bulb socket. It gets all the information from one and the other is not needed. I am guessing that this setup would be similar to what you are describing. From your experience, do you think that this will work? Have you had any problems with your setup? The filaments on the factory bulb look the same size. Do the factory lights use dual elements for hi beam? One more. Have tou had any problems with the inside of the headlight reflective surface melting or burning or getting discolored? I have heard this elsewhere. Here is a link to the set I have. Looking for a voice of experience here.
http://www.vvme.com/