NOOB questions 2002 turbo loz lancer

Hey guys i am thinking about buying a 2002 lancer oz rally edition. it has a garret t28 turbo and FMIC 470cc injectors currently "tuned" for 7.5 psi. It has an evo front end conversion and tien s tech drop springs progress front and rear sway bars and strut tower bars. the car looks beautiful! thier are a few things i noticed that have me a little scared and want to get everyones opionion on. First off it has an evo gauge cluster that reads in KMH which is a small thing but since the cluster is not stock how do i know the mileage is accurate. i am a GM mechanic and i know mileage on those is stored in the cluster. is this true on lancers or is mileage loged in the pcm than sent to cluster via the bus? ALso the car is "tuned" with an apexi SAFC which i've used before and works great for tuning an exhuast and intake but i'm not to certain on boost applications. the car idles at 1800rpm than will drop down to 800 than jump back up any idea what is causing this? also the car hesitates or has what feels like a mis fire but clears up on accel. i think the cause for that is the boost is turned down to 5psi but apexi tune is still set to be running at 7.5 so its possible its loading up. it also has a rising rate fpr would it be better to ditch that for an adjustable and run a 155 fuel pump? thanks alot for all your help guys.