It should be fine if you do the cat first. You'll get a boost of energy for a while until the ECM learns down.
Two other mods you MUST do (unless your car already has them) are the DDM backbone and probeam. TRANSFORMS the car's handling (and safety in the corners), especially with more power.
Owner of "Campbelle", a Brazen 2008 GXP ... with mods piling up...