Any just to add to this, I tested with SB fully OFF in the following configurations:
2.4Ghz, WPA2/AES Only
Xbox 1 connected to a DIR-868L in bridge mode - BO2 game status reported Moderate NAT
Xbox 2 connected to a DIR-865L in bridge mode - BO2 game status reported OPEN NAT
Switched to 5Ghz, WPA2/AES Only
Xbox 1 connected to DIR-868L in bridge mode - BO2 game status reported OPEN NAT, game play was 3 yellow bars.
Xbox 2 connected to DIR-865L in bridge mode - BO2 game status reported OPEN NAT, game play was 3 yellow bars.
Switched to a DIR-857 as main host router.
Xbox 1 connected to DIR-868L in bridge mode - BO2 game status reported OPEN NAT, Will test game play status later.
Xbox 2 connected to DIR-865L in bridge mode - BO2 game status reported OPEN NAT, Will test game play status later.
Both bridges are reporting 450Mb Wifi connections on the 857 status page.