There are clutch kits for both autos and manual transmissions, these will help get the power to the ground better and help in turning larger tires.
As far a just a jet kit, its a wate of money if only added alone, the gains just aren't worth the cost. Now add that with a less resrictive air intake (snorkles, K&N, etc) and exhaust (mod the stocker, aftermarket) then you will see some gains in power. Other then that your somewhat limited in performance mods without spending big money.