A certain percentage of your allocated bandwidth will be used for overhead and error correction. It's extremely rare to reach the full theoretical speed.
Cool, thanks for all the feedback!
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The Speedtest.net servers can't keep up with my test :P
It shouldn't, unless you have less than a 100% clear signal. Your router should be connected to your modem with at least a 54Mb transfer rate, so it shouldn't slow down much.
Where you probably could have a speed problem is ping time, which is really only a concern if you're a competitive gamer, where ping time could be critical (the difference between a headshot and being shot).