Unfortunately there is not much to be had out of the M54. Being a 5 series you probably don't want to do Exhaust mods(headers,) which is where you will pick the most up. Intake/tune will get you maybe 5whp. Not owning a five series I can't really comment on suspension mods. If you want a sporty look inside consider wrapping your trim pieces in 3M carbonfiber wrap looks great!
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Formula SAE "Speed is Sexy"