BMW's are well engineered pieces of machinery, out of the box there is very little to be had over what the factory already brewed as such tuning becomes a lot harder then sticking a fart can out back and picking up 50hp.
our cars are pretty much maxed out and short of forced induction its extremely hard to make significant power, also R&d costs are very high and only a handful of tuners have a clue on how to properly tune these cars.