if you upgrade to a 2.5 header from a 2.25 and upgrade the cat, and just leave the 2.25 exhaust is there anything to be gained?>?? or just leave it as 2.25

Current mods are micro V pulleys, aem short ram intake, obx 4-1 , test pipe, 2.25in catback, and sun auto hyper gauss
i put a nice dent in my header today thats why im asking.

one last thing is it alright to run 93 in my motor becuase the gas station closest to my house serves 93 instead of 91?