I think your tire pressures being switched is because at some point the tires got rotated without relearning the sensor positions... 08's dont have the passive system, they have an actual sensor in each wheel assigned to a particular position. When you move tires around, the sensor reporting for, say, Rear Left will still report for rear left, even if you moved it to front right.
But yeah, replacing the HU only loses steering wheel controls and DIC calendar functions. Everything else is retained as long as you get the proper interface, I use the Metra GMOS-04 with zero issues.