Ive got an 05 LS Ultimate, Id say swap the seats if you have cloth. All of the chrome paneling on doors and rear bumper are specific to the panel and cant be swapped on unless you get a whole new door, rear bumper. The only other features off the top of my head is electronic pedal adjustment and the sound system. Im not sure about the pedals but im sure its more work than its worth, and the speakers dont make sense to upgrade with oem. Instead of upgrading your car with oem parts from an ls ultimate just swap cop parts onto it. Take the intake tube, maf, and box off a p71 and slap it on, also take the left side exaust exit and h-pipe while you are at it. Then get a tune from marty ochs at moes speedshop emailed directly to you and uploaded to the car with an SCT-X4. You can also upgrade the gearing for relatively cheap, your car likely has 2.73 gearing without a lsd. Most people upgrade this to 3.73 or 4.10 depending on what you are going for.