Nothing, which will give you the same 'bang for bucks' (as the saying goes) compared to your BMS JB1. You could, of course, team it up with a performance exhaust, but from what I have read, 'performance' is a bit of a misnomer.
Sure you might get a few whp and extra torques, but the performance part is mainly vocal performance from the exhaust note. Nothing to scoff at, but not going to enhance acceleration performance terribly much, if at all. Not really designed for it.
There are intake upgrades, but again, not much in the performance gains there either. However, teaming all these upgrades together should add to the seat of the pants feeling of extra overall performance.
Don't forget to use the best RON fuel you can get your hands on.
328i M-Sport, Estoril Blue, Carbon XP 35 Tint, front heated seats, black Dakota, Sports Suspension, Anthracite Headliner, HUD, Pro Nav, Sunroof, hk 600w, Bi-Xenon, 19" 403M wheels, Reversing Camera, DAB+, Xenon, BT Ext Connect.