400a package gets you:
heated/cooled leather seats
My Ford Touch (with the 8" LCD touchscreen)
the toggle switches with selectable driving modes
dual zone climate control
heated mirrors
rear diffuser
9-speaker stereo
other stuff like ambient lighting, aluminum pedals, etc... I'm not sure if that's included in the base GT or not?
out of those, I'd want the 9 speaker stereo with the lcd display and rear diffuser...