The numbers are measured using 1518 bytes because large packets provide the most attractive numbers for firewalls. Cisco does not publish any other numbers. I've seen other vendors compare firewalls to the Pix using UDP packets of different sizes.
Of course, even those numbers aren't achievable in a production environment as you'll have more than one host talking, TCP states to maintain, more than one process running, etc. If you really need to sustain more than a couple hundred megabits consistently, I would look at the 535 or a hardware solution like the FW blade in the 6500s. Also, Netscreen has some screaming hardware firewalls. [ sorry cisco ;) ]
