A moderately modded SD car can develop A/F ration problems, and Idle problems when vacccume is affected..with addition of cams to aggresive for Sd to adapt to the change. You can change heads and intakes along with most bolt-ons but all but specific cams will give you problems. Mod away all you want untill you start to have issues then consider converting..I had all the mods in my sig and only problems i really had was idle related..only reason I am converting is my desire to go with heads and cam with fuel system upgrade..converting will prevent possible issues that these mods would create with an SD car ...don't change untill you need to, the SD will adapt well to most anything you want to do with exception of CAM and injector change.