You could get 375 rwhp for $3500 from a high flow cat midpipe, tune, filter and pulley, retaining the stock exhaust. Or you could get 350 rwhp for $3500 from a slip on rear exhaust. I think Akra quotes about 3 rwhp for its slip on rear exhaust. BMW quotes "a more powerful sound."
Do you want to sound good, look good, or go faster?