Expect anywhere from a 10-20whp gain.
ITR heads are mildly ported, they have more agressive cams and better valve springs.
Your GSR intake manifold wont bolt up(I think so don't quote me.), and you'll need an ITR or skunk2 manifold. If you require a Skunk2 or ITR manifold, you will also have to use a chipped obd1 ecu, or an ITR ecu from your year car.