After a lot of challenges (most of them due to my ignorance) I have finally gotten my DIR-655 and Lenovo x301 with Intel 5100 AGN adapter to connect reasonably reliably at high wireless N speeds (243-270Mbps). Intially I couldn't get above 54Mbps. Once that was figured out I found connection speed fluctuating wildly. Took me a while to get this reasonably well addressed by changing some settings on the adapter (e.g., "roaming aggressiveness", "ad-hoc power") and some on the router (e.g., turned off GI something). This seems to have gotten performance to a pretty good level although speed fluctuates a bit still. When I run speed and QoS test at ISPGeeks.com, QoS stats are still not as good as on one of my other computers. This manifests itself in real life through occasional stuttering of streaming Internet video. There is really no reason this should happen with the hardware I have.
If anybody has some ideas around settings/tweaks (router or adapter) that stabilize computer-router speeds, let me know.