So if I replace all with 4 ohm infinity speakers. Would i have to replace speaker wire and and factory amp?
I think the stock wiring is 16ga (someone correct me if i'm wrong, i haven't actually looked). The 16ga is probably good for 18amps. i don't think you'll touch 18 amps even with a power upgrade.
Like others said, it might be a good idea to just swap out the door stockers with a decent upgrade and see if it gets the sound you want. If not, then look into an amp.
If you're looking for real high quality sound then you might as well tear everything out and start over. I'm big into high quality sound and the reason i havent touched mine yet is i really don't want to rip everything out. I have the nav w/ sync and i dont want to lose any of the nav/sync features...i'm actually quite happy with what Ford provided outside of the sound quality.
2012 Mustang GT Premium Kona Blue W/ 401A Trim
Electronics PKG, RR Sensing/Security PKG, HIDs PKG, Comfort PKG, Brembo Brake PKG, Glass Roof, Rear Camera