hey guys

im about to put in an amuse header to replace my stock one.
would u guys happen to know if a new cat is required or not?
would the amuse header still be compatible with the stock cat?

i've been looking at SARD cats. any good?
dimensions are 76mm in and 60mm out.

if i was to put in the header...would i require a tune?
i know everyone says that u need a tune to maximise its use, but i just wanna put it in coz i've got it already.
would it decrease mileage per tank? (drastically or minimal?)

cheers!