I've just spent the last week doing research on a new in-dash Nav/DVD/MP3 type unit. The problem that I heard with the JBL Synthesis is that you can get feedback from the engine through the AMP or something like that. However, I've spoken with three different professional installers in my area and none of them seem to be concerned about getting my new unit wired correctly. I ended up getting the Avic-Z2 - found it on-line for an unheard of price of $1339 -
www.thecameraprofessionals.com.
I'll be getting the install done next week and can let you all know how it goes.
I agree though, why pay the money for the higher end HU if you're just going to rip it out unless you want it for the speakers - it would probably be cheaper and better to get aftermarket speakers if you're looking for more sound out of the system.
My next 4runner will have the cheapo factory system and then I'll spend the money on an aftermarket system and will sure get more for the money that way.