The CSII is no big deal. The system does not have an amplifier. The radio has it's own outputs just like an aftermarket unit. The only thing "unique" about it is that the front door speakers are wired in Parallel, a woofer and a tweeter. I can speak from experience and say that aftermarket speakers won't benefit from the factory head unit as much as they would from an aftermarket one.
The wiring in the car is more than enough to handle aftermarket speakers unless you plan on running 200W a speaker. All I did was cut the factory connectors and add my own.
To change the door speakers, all you have to do is remove the plastic grille from the door panel....no need to remove the panel itself. The rear speakers are removed from inside the trunk.
As far as wiring up some subs.....
The battery is under the rear seat, which makes for a gravy Amp install. The only "hard" part is running the RCA's from the new head unit to the Amp.