I've been "considering" replacing the speaker in my 12 MKZ as well. Overall, the system sounds okay at lower volume levels but there is definitely a point where the sound starts to crackle at the higher volume levels. Personally, I wouldn't spend $300+ on Pioneer/Kenwood speakers as I don't believe they will make that much difference over the factory speakers. Yes, the sound would be somewhat better but is the cost really worth it?
"If" I actually decide to replace my speakers, I'll be replacing them with Rockford Fosgate speakers. I'm debating the addition of a sub enclosure in the trunk. I already have Orion HCCA amps I removed from my Ford Expedition but I'm asking myself... do you really want to go down this road again?