I purchased an installed a pair of Infiniti Kappa 682.11cf speakers and the direct 8" sub replacements from Shelby/Kicker from the Shelby Store. The sound is far better than the junk factory speakers.
The only difference is that I must have misread the speaker specifications from Crutchfield. A post under their Mustang install page mentioned that the front door speakers should be 2Ω, but the customer in question had a 2014 Mustang
GT. Mine is 2011 and when I pulled out the factory front speakers, they are 4Ω. Either the information in the Q&A was wrong or Ford changed the specifications in later model years. Regardless, I am happy but will have mind the volume...I don't know the limit of the stock head unit in regards to current drive for the speakers.