Step one, tune up with new cap, rotor, plugs, wires and air filter. Hard to get gains unless it's running good. I guess your S is drawing it's air from a cool location now, I know my 8V 944 does and so does my S2. Get a good chip, don't risk your engine's health with one from EBAY for $39. The S suffers from overly aggressive knock prevention programming which holds the timing retarded for longer than nessciary before returning to normal. This was corrected with the S2
Step three, the factory headers aren't too bad in design so a high flow catalytic converter and muffler would help too at a fraction of the price of a header. The above should net you at least 10% over the your engine's current state of tune..