Change 120MPH speedo into a 200MPH speedo?

I always come up with the weird ideas...
I have a 411 PCM from a 2001 Suburban.
You can get or create custom instrument cluster faces, that's the easy part, but can you adjust the numbers in the PCM where the normal sweep of the speedometer would be a 0-200 MPH range instead of the standard 100 or 120MPH?
This would be using a 5.3/4L80e combo without ABS.
The general purpose of this would be to save $400 on an aftermarket speedometer if someone's racey street car needed a higher limit than the gauge provides.
My guess is that it's possible since calibration is an option... but what are the limits (if any), and what could those changes affect?

Thanks in advance!

- Jason