OK, interesting development.
I reset my DIR-655 to its factory defaults just to try something new, and after setting up the Internet and Network Settings I got around to setting up my Wireless Settings again. This time i started by leaving the 802.11 Mode as mixed b/g/n with Auto Channel Scan enabled, Transmission Rate set to Best, and Channel Width to Auto 20/40 MHz. I left the WPA Mode set to Auto(WPA or WPA2) and changed the Cipher Type to AES (since that sticky about 300 Mbps wireless N speeds says it is required); so basically the settings were almost exactly the same as before. Here is the speed test result:

No luck, same poor speed results as before. After pulling out a little more hair I decided to try changing the Cipher Type. I changed it to TKIP (instead of AES or TKIP and AES) and restarted the router. After setting up the connection on my laptop I ran a speed test again. And...

Bingo! I'm now able to watch high-quality streaming video on my laptop with no stuttering and excessive buffering like before. For whatever reason, apparently the AES cipher doesn't agree with my wireless adapter. The adapter properties in Windows list the speed at 54 Mbps now, instead of 300 Mbps like it did before, but my Internet speed is three times faster.
Unfortunately, my network transfers (copying a video from my wired desktop to my wireless laptop) are slower than before (Windows copy progress window averages around 2.3 to 2.5 MBps, while it was around 6 MBps before), even though the Windows wireless utility shows that my router is providing an 802.11n signal.
I'm wondering now if anyone has any ideas for how to increase my network transfer speed without sacrificing the great Internet speeds I am now getting over wireless, if that's possible. I guess I'm curious if this development with AES not working properly clears anything up.
EDIT: OK, after further investigation it appears that it might not be a cipher issue, it might be a problem with wireless N itself. It looks like I am getting the best internet speeds on my wireless laptop when connected via wireless G, not N. Now I'm not sure how to determine if the problem is with my laptop's wireless adapter or my router.