1. Boost gauge: turbochargers are known to be subject to overshooting specified boost with rapid throttle position changes. With a boost gauge installed you can monitor this overshoot and control it by backing off the throttle. In addition a boost gage is one of if not the best engine performance monitor around.
2. CAT 2 Micron filter is as it states a 2 micron filter which means it filters out a high percentage of particulate down to 2 microns. The superior filtering, OEM 10 micron, will help lengthen the life of the injection pump as wear will be lower.
3. PP520's and RCII: ooh tough one as it makes the car very nice to drive but is problematic from a dealer perspective as it is not their ECU nor injectors.
What problems are you having with the car?