After much research and experimentation, I have optimized my 802.11n channel width. I have set it to the unconventional, yet best performing setting of 5 MHz.
I live in a busy city, and often count more than 15 visible access points with in range of my home. All these access points compete with mine - they share the same spectrum, and if they're talking at the same time my wireless router is talking, error detection and retry protocols kick in.
802.11n has a standard width of 20 MHz, and optionally can be expanded to 40 MHz. This improves performance, right? Well, not exactly.
If you live in a WIFI void, with no competition, 40 MHz is awesome. But in the real world, some other device chatting within that channel is going to result in significant clashing, resulting in performance-killing error correction protocols. So I took the counter-intuitive approach and brought my router to 5 MHz.
But 5 MHz is a lot smaller! Yes, with a narrow channel your theoretical bandwidth slows down to about 1/4 the performance. But it isn't a theoretical world. It's a real world, where there are competing radios.
With a narrow 5 MHz channel, it is much less likely that a neighbor's access point will compete with mine, and therefore error correction protocols rarely kick in. And the proof is in the numbers: with a 5 MHz channel width, I average about 16 Mbit/sec - about 10% faster than when I have my access point set to 20 Mhz, and faster than my ISP in any case!
Of course, your mileage may vary. If you live in a rural setting where there are no other access points, 40 MHz is likely the best choice. A suburban area with houses every 100 yards apart might be better off at 10 or 20 MHz, depending on the local airwaves. Your best bet is to try it, and measure the performance.
Another advantage of 5 MHz? Longer laptop battery life! The wider the channel, the more power your laptop has to expend to transmit on the channel.