My Grand Am had the ultimate sound system in it for my liking. And it took the breath out of you at the same time, literally lol. It would never fit in the Mustang and I just don't want it in the Mustang. It's in the Jeep. The Mustang has a... I don't even remember the brand 12" in it with about 500rms going to it. It's a 250rms sub lol but I know how to tune it so it doesn't blow up on me. Plus I never have the sub all the way up either. The Grand Am had this:
2 American Bass XD 15's in ported box
3000rms going to them from an RE Audio amp
Rockford T400-2 powering the highs
2 P1 6.5's
2 P1 6x9's
If your going for a good system in your car and don't much care about the added weight DEFINITELY add dynamat. Dude the subs rattled my Grand Am to almost pieces because I didn't add any. The wing was starting to crack because of the vibrations being so violent on the trunk deck. I can't stress that enough. A good system is nothing without dynamat.
As for the highs, well once you dynamat the car throw some Rockfords in (you can use whatever brand you like, but Rockford was always my favorite for highs and amps) and see what it sounds like. With the matting in place, you may not even need subs to satisfy your bass needs. The matting will help give you more solid bass without all the rattling and such that causes you to lose some of the lower frequency notes..
Whatever you choose to do tho, I'd be sure to ask
ttocs his opinion. He's our resident ex audio installer and he'll have all the answers for ya