the biggest difference is removing the standard cat and manifold in favour of a 4-2-1 or 4-1 manifold, you will have to swap back though come mot time unless you have a friendly tester. coupled to something like a milltek exhaust you should see upto 15bhp more.

insuction side don't go for an open cone unless you relocate the battery to the air box and then possition the cone undernieth the batteries original location. then its cams and remap unless going for throttle bodies or forced induction.