well our cars don't do a replacement "chip" per se, but software. but I got what you were asking. the M52BTU (which is the 323) does respond better than the 325/330 to software. but still nothing huge. you can get a custom tune or generic tune from Conforti SHARK INJECTOR. intake & exhaust aid the gains of the tune. underdrive pulleys help a tad but I'd save that for last.
you can do a lower geared diff ratio like 3:46 that helps performance, although nothing to engine.
OE CSL WHEELS - OE CSL BRAKES - OE AUTOFLD MIRRORS - CSL RACK - CSL TRUNK - CSL DIFFUSER - AA TUNE - AA PULLEYS - AS 40% SSK - BILSTEIN PSS9s
CF CSL LIP - NAV- BMWP CF STRUT BAR - BMW GROUP N MNTS - GRUPPE M CF INTAKE - SUPERSPRINT - UUC S/S LINES, RTABs - H&R SWAYS - 4.10 M-SPORT DIFF