I installed a non-navi radio (from a 2006) in my navi tsx (2008) today. There is a black connector for the navi unit that is not used but everything else plugs right in. The clock screen above the center vents does not work, it shows a garbled mess. You can get the non-navi clock or just leave it unplugged like I did. The non-navi unit doesn't do voice commands so the voice command buttons on the steering wheel do nothing now and could probably be removed.
I plan to modify an aftermarket unit into the stock one so I won't bother replacing my clock screen since the aftermarket unit will have a clock. I also don't think I would be able to set the clock once I remove the stock radio controls. I am going to wait and see if the aftermarket unit with a steering wheel controller will let me use the voice command buttons, if not I will remove them.