To answer your question - yes, only the stock head unit will retain the ability to send signals to your instrument cluster.
The big question is really what functionality you want from your head unit. If you just want to be able to run amplifiers, and have an auxiliary input, then Phreddy's mod is definitely worth considering...particularly since you're obviously concerned with maintaining stock functions.
If you're after additional functions like bluetooth, DVD or MP3 playback, amplifier control, time correction and equalisation then you're not going to be happy with mods to the stock head unit. You will have to do without the instrument cluster readouts though.
The rest of the choice is up to you. I understand some aftermarket head units (Alpine and/or Clarion I think) have direct remote inputs that are compatible with factory controllers. Less compatible head units can receive signals from the CAT09 or CAT10 either via a wired plug or IR.
I'm using a CAT10 with an aftermarket head unit and I'm quite happy with the results...I get the functions I want and I can still use the factory steering wheel controls. I didn't miss the instrument cluster readout for more than a couple days - I have a 7-inch TFT colour screen for that kind of thing
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