B16Z6+S20

ok, before i make a mistake like replacing the stock cat on my GS-R, i thoug i checked before making important changes, so:

- how good is the stock muffler on a 99 GS-R?
- what benefit i can get (as in HP, TQ, MPG, Etc.) if i replace the stock 2butterfly intake for an stock SI manifold?
- are stock 96 ITR springs harder or sportier than 99 GS-R?
- and finally for now, what is the wire that sends the RPM signal to the gauge on a OBDII P72 ECM?

thanx.