firstly, premium sound, pfft! crap ass sytem from ford, better then average, your actually gettin 13RMS from the unit to each speaker itself, claiming probably in the 40-60w rms it says on packaging and stickers etc! :) (found this out thru readin carefully thru it etc!)
that was on the ef/el's! .. the new ones, well im guessing its not too much better!
will giveyou an *IDEA* of what you can get, the prices arent to bad, but if your interested in shopping for car audio, MLC in liverpool are fairly good.
2 x 10" clarion/jvc/blaupunkt subs (dual voice coil or single)
( why10's? less weight, punchier then 12's.)
pioneer horn 6" split (new one they have, very nice!) or jvc/blaupunkt OD series)
front, once again, jvc/pioneer or clarion (Even alpine).
running 2 amps, a 4 channel amp for the 4 speakers in the car .. at least 50rms x 4, and a 2 channel amp for the subs :)
Thats just personally what i'd do and have done before several times and always sounds right and daaaaaaaaaamn loud!