Zero performance gains, better looks maybe, potential for a hotter air charge and actual power LOSS if its a crappy one.
The newer "CAI/TUNE" packages that are sold are kind of... misleading. People experience these big power gains after installing the CAI and flashing the tune but everything they are feeling is from the tune.
edit: If you are thinking of installing a CAI without a tune then don't, you'll **** up the stock tune and it'll run like ***.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.