Not only will you need a tune but you will also have to upgrade your fuel pump. I suggest the SVT focus fuel pump which runs at 255l/h.
Do the CAI at the same time (it also requires a tune).
You'll find that it's difficult to find an air intake to fit with the stock supercharged saleen. Chicane makes one...but it's not a cold air. Most CAI's dont' fit the series I and series II superchargers. JLT's CAI only fits the series IV S/C.
What he said... If you don't upgrade the fuel pump, your motor will be on borrowed time.
Your A/F is probably borderline already...
Mark at Performance Autosport also has a CAI that will fit, it might be the same one that
Joe at Chicane sells, I don't remember. I have to order one for my 99 now that I have
the bigger pump and 66mm pulley. You might also want to look into the MoSaleen pulley since you
will most likely get belt slippage with the smaller pulley. It also eliminates the belt jumping
that usually takes out the upper radiator hose. Check the bottom of your upper radiator hose,
you may already have some damage to it... most people don't know about it until the hose lets go....
Mike and Dwayne at Big Daddy just did all the upgrades on my 99... completely different car now....