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.
OEM CSL WHEELS - OEM CSL BRAKES - OEM AUTOFOLD MIRRORS - CSL TRUNK - CSL DIFFUSER - AA SPORT TUNE - AA PULLEYS - BILSTEIN PSS9s - AUTOSOLUTIONS SSK
CF CSL RACE LIP - NAVI HU - BMW PERFORMANCE CF STRUT BAR - BMW GROUP-N MTRMTS - UUC SS BRAKELINES, TRANS MOUNTS, RTABs - 4.10 BMW M-SPORT DIFF