Are you looking for convenience features or sound quality? Swapping head units will do nothing for sound quality and very likely make it worse, you have to run a good amp and change speakers and subwoofer to improve the sound quality. Top of the line for Gen 4 head unit is RB1 and with the other upgrades, it has the audio output sound quality comparable to anything else on the market. If you want a different head unit due to features, any of the 2 DIN units will do if you modify the opening although you will run into issues regarding 2 way front component speaker and subwoofer configurations (rear speakers you can ditch in any case). To get the audio upgrade done right, you either have to be competent yourself or find a competent shop (not a usual hack shop). If you have no serious audio experience, at least IMO, best to leave things alone.
If I recall from my little project, all signal processing, except for fade and balance is done through the OEM amp. If you do a head unit swap, this is pretty basic stuff and wiring harness is available. Pioneer and Alpine make very nice GPS units, you pay more but you get better reliability and quality. There are Chinese units that are direct swap that do not require opening mods but the screen is smaller and it is what it is... With aftermarket, you will lose subwoofer controls since OEM amp does not account for it. If you have a coupe, you can add an extra amp for subwoofer (more room than drop top) but this will require running wires from battery and from HU to that extra amp.
