The coaxials in the doors and rear deck have a 4 ohm impedance.
The 8" subs in the doors are DVCs that run at an odd 1.8 ohm impedance (according to things i read on other forums). Personally, i wouldn't think there would be a problem running DVCs at 2 ohms or even an SVC at 4 ohms in place of the OEM subs.
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