there's not too much u can do... big car, small motor...
if you do decide to go with forced induction it'll be faster, but you should have really started with a bigger motor especially with that size car...
so what you need to do is find a capable shop that will either A) swap in a V8 or 3.5L six and/or B) turbo or supercharge the V8 or turbo the 3.5L six.
be ready to spend big money to make that car fast.
I wouldn't even wast u'r money on a chip, exhaust, etc for u'r current motor.
That motor is slow in a 3 series, never mind a 5er.