Ok so setting my 5Ghz radio for single mode AC, WPA2/AES only, auto chl scan and using a bridged DIR-868L connected to the 5Ghz SSID with 18ft+- distance between 5500 router and bridge semi line of sight with no walls, streaming 800Mb Mp4, 4Gb mkv, and raw 9Gb DVD files seems to work well and doesn't buffer when streaming to a Boxee Box. However raw BD files are buffering every 20-30 seconds and the 5Ghz can'e seem to handle this kind of media. The only routers I was able to use with out seeing much if any buffering with using raw BD files was when the DIR-868L was host router and a DIR-865L was in bridge mode.
I didn't watch an entire movie however I did stream at least 5-10 minutes with out seeing any buffering issues using MP4, mkv and raw DVD files. FW loaded currently is v1.11B03.
DNS Relay is OFF and manual DNS IP addresses are being used.
Update: I watch a raw DVD concert via 5500/868L bridge most of the way thru and didn't notice any issues.