I had my 2001 M3 up to 160 in 5th gear. I thought the limiter would kick in at 155 but for some reason it didn't.

thats unusual - but bmw speedo's have been known to round up as much as 8 mph at 100mph and more at higher speeds
ie
it says 100 but your really doing 92

maybe you hadnt officially hit the real 155 yet