i thought MY98s ran rich, but have been told they run lean, i have some soot on my exhaust, but not too much... anyways...
you can use electronics to lean it out, like an S-AFC, it's just tuning... putting on an intake might help, more air getting in there, but i think electronics are the way to go if you want to fine tune the a/f ratio, not that i've done anything to my car, so don't blame me if you lean it too much and the engine dets and dies as a result.
meaning: if you do lean it out with some electronics or something, don't lean it too much, it can hurt
there are reprogrammable ECUs (TEC II, and some others that i don't know (haltech maybe))as well where you can tune this as well as many other things but if all your worried about is soot on the muffler then i doubt you should go that route =)
ooh ooh, second edit, leaning out the ratio to stoic from rich gains you some power (don't know if minute or gargantuan or in between, but it does)
third edit: i was looking at a site once that had listed the wire colors and such for wiring S-AFCs to the RSs, but it said that APEX'i hadn't released wiring specs. for the 98s, but im sure some 98 ppl here have done it, so i'd ask around if you were to get one.