The myth comes from two places.
One is from Dinan through me. I asked my Dinan tech advisor ,and he said Dinan tested various rear sway bars, and the car was fastest with the stock bar.
The other place it comes from is forum members who tested the M3 swaybar and got so much inside rear wheel spin, they either put the stock bar back on, or (at least one) removed the bar completely, or added a limited slip diff.
Some people have tried the 335ix rear bar, which is just slightly stiffer than stock 135i. It was only two or three people who tried it, but IIRC they liked it.