I did the rears in my Golf.......Dazza is right! just do the fronts and a sub....Plus spend as much money on the front speakers as possible. I used the ESK's and they simply weren't good enough (to bright and quite harsh sounding)
Also, when looking at amps choose quality over power. I chose a digital Alpine amp and I've suspected ever since that it contributed the bright sound.
I used my RNS-510 and sent the signal to an Audison Bit Ten. Fabulous piece of kit but absolute overkill for what I wanted to achieve so I would also recommend just a simple line converter. It would be cheaper and far easier to configure.
All that said, although I probably wouldn't go to the same extreme again, I would definitely consider adding a simple sub/amp to my CC.
I'm going to do the rears as i now have 3 sets of speakers. Will fade them out probably.
I currently have the HSK's in the drivers door and ESK's in the passenger door (Both Not amped). The HSK's do sound better.
I may amp them both up before i switch out the set in the passenger door.
@Biano44 you have me thinking now maybe i should buy a better amp.