Here...
"Some users have been reporting performance gains when using two single bridges with their 1080 SLI setup. To my knowledge this caused issues with the Maxwell based cards but I decided to give it a shot after completing all the testing previously. To my surprise with two flex connectors installed I received the exact same boosts in The Witcher 3, Division and Fallout 4 as I did with the high bandwidth bridge. So it seems if you have two legacy connectors you don’t actually need a high bandwidth bridge."
Basically the HB Bridges are just two normal SLI connectors. I figured that for a while but never found any proof.
There is no "rating" per say, just if you have 1000 series in SLI you will benefit from the HB Bridge or using two "legacy connectors"
It also seem unless you are playing at 4k, it will no zero improvement over the old single connector.