The obvious alternate method of troubleshooting also occured to me...that I can hardwire a mac/PC to the modem and run a few tests WITHOUT the AirPort Extreme.
The way to do this would be first turn off the wireless on your MBP and connect it to one of the LAN <-> Ethernet ports on the AirPort Extreme using a known, good Ethernet cable. If the speed that you see there is around 90% of your service plan, then you know the AirPort Extreme is working correctly on Ethernet. That is because there is always some speed loss in the routing process.
When you turn the wireless back on, you won't get close to the Ethernet speeds unless the MBP is very close to the AirPort Extreme. Even then, wireless is almost always going to be somewhat slower.
The speed over wireless will decrease as you move the MBP further from the router or the signal encounters any obstructions in the direct path to the router. This is normal.
We haven't talked about wireless interference, and that is always a possibility unless you are lucky enough to live in an area with no other houses around for 300 feet or so. Even then, cordless phones, TVs, baby monitors, security systems and countless other devices can still affect the wireless signal.