"After some research it seems that there are two answers here...
1.) The ~20% Limit
The 54Mbps is, for the most part, a theoretical upper-bound that will never be reached in a real world situation. If you can get half of that limit you are doing pretty good for yourself. If, like me, you are coming up short there are a few things you can try...
Change your wireless channel. You may find less interference and better performance on another channel. Reposition your router/adapters. Things like walls and metal objects can cause problems. Sometimes a small adjustment to the location of your devices can increase the speed significantly. Use the same brand. From what I've read, people seem to have much better performance when the router and the network card are from the same brand.
In my particular case, moving the wireless USB adapter seemed to have the most profound affect.
2.) The Media Player Usage
I don't have a good answer for this; except to say that Media Player might be improperly configured or have a bug of some sort. Using other players, like VLC, yielded a much more consistent (and much lower) usage. I'll be upgrading to the latest version of Media Player tonight and seeing if that helps or not; but VLC streamed the content and used an amount of bandwidth that seems to align with my mathematical prediction.
It's possible that Media Center is trying to download more of the movie at once to build up a large buffer or something; but I can say running two instances of Windows Media Player resulted in lag. I ran four instances of VLC and it was lag free."