As far as I know, 150Mph is the only optional upgrade. That limit is only available with the 20s as stated earlier. It's all about the tires.

A lot of companies do this, they just do not make it public knowledge the way BMW does with this option. For instance, my SS has a top speed of 162Mph (fuel cutoff) whereas a standard Z28 is 140 or so, and this is soley because of the tires that came on the car.