Just dynoed a 1995 m3 with a very popular chip tunners chip and made 3 pulls. Took out the chip and made 2 pulls.
There were several areas where the chip runs had 2 to 4 more hp than stock, and at 6500 it was down 2 hp with the chip. I will still leave the chip in since the car seems to run smoother. I know every car is different, and I have seen cars that had custom chips burned on the dyno and they have a better result. I was supprised since the tunner advertises 18 hp, not even close. My .02