I've heard BMW speedos indicate, at that speed, around 8 mph above your actual speed. I haven't researched the claim, but that's what's floating around in the top-speed discussions on other boards. As for the wheels, did you get them put on or did you buy the car with 17's ? I only ask because wherever you would have bought from should have explained plus-sizing, which is the use of a lower-profile (sidewall) tire to accomodate the increased wheel circumference. There are calculators out there for this sort of thing, but basically if you go from a 205/55/16 to a 225/45/17, you have the same tire circumference (enveloping the wheel), so your speedo and odometer are correct. Likewise, a 245/40/17 tire on a wider 17" wheel (what I have in the rear) has the same circumference as a 225/45/17. Bottom line is that if your 17" setup is correctly matched and plus-sized, your gauges are as correct as they came from the factory.
top speed - unless you ran out of gear (tach. was pegged at the rev limiter - shouldn't have happened although I could be wrong), then you probably hit a point where your power couldn't overcome the drag anymore. So you either need to decrease drag, increase power, or both. Go for more power!!