Check out the Audio sub-forum. Your options are basically null as far as upgrading the HU, it is integrated with the HVAC controls. So your only option is the JDM unit.
Otherwise you can use add on stuff to upgrade the outputs and run accessories, or get a din cubby for above your stereo and go from there....
Unfortunately, the HU on these cars is a major fail if you have any interest id upgrading
For me, I got a stock sub and XM unit and I'm content. But I'm no audiophile