Your car's spec's sound like mine (BF00199), which is a 535is Australian delivery; 5 speed manual, LSD, lower suspension, 'Recaro' style sports seats with better leather, M Technic bumpers and skirts. A great car to work up. I'd recommend a Miller MAF which comes with a chip to replace the 'barn door' air flow meter. This made a sinificant improvement to the mid range pulling power on my car.
See Miller MAF These are great value. Don't expect any better fuel economy though. I've put newer style fuel injectors in too.
Also ditch the twin mass flywheel and replace with a lighter flywheel and clutch from an earlier 735 M30, eg E23. I've mildly ported and polished the head on mine for a slight improvement, but the stock M30 heads flow pretty well.Before the Miller chip my car had a PowerChip Gold, but the Miller chip brought improved performance.

Have fun! Colin.