an orp will get you more power than the high flow cat, but may not sound ideal on a factory exhaust. a CAI is virtually pointless; a k&n panel filter in the stock airbox and the silencer (S shaped tube) removal will get u the same power, almost the same noise, and wont expose u to hydrolock!
beyond those, there is decking the head (or installing a regular zetec head gasket; thinner), cams, intake manifold...and by this point turbo becomes your best bet.
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