In addition to the the Alta intake, 15% pulley, and Magnaflow exhaust already mentioned my next fav's are MUCH lighter wheels, better tires, and my Texas Speed Works chassis brace.
Lighter wheels reduce unsprung weight and improve handling and ride a bit, even accelaration and braking a bit. Very open wheels like the defunct K1's my car wears half the year are under 17 lbs for a 17 x 7 and are so open it even helps brake temps. Ceramic pads make a huge difference in reducing brake dust, especially important in a cabrio as it comes in all over the back seat!