What I think everyone is skipping in the estimation of Guardians is they will almost NEVER be assaulted. Take a 20 man guardian squad in defensive fire. Forty Shots. 7 nearly guaranteed AP:2 wounds. Think about that. So, they have a 12" range. Think of flying daemon princes. One round within 12", dead. boom. Tries to assault, dead, boom. I'll take that!
I think your math is off here... You still need 6s to hit in overwatch. Then roll 6s to wound for AP2 (if I read bladestorm correctly, it's just a castrated form of rending). So 40 shots will yield 7 or so hits. Of wich 1 can be expected to be AP2...
I think guardians still are the worst troops in the game now. They are still shit sandwiches... just not soggy ones anymore.
They are going to suffer form the same thing they do now: 1 shot round of actual shooting, maybe two... and then ground into a bloody pulp if that damage wasn't adequate. Completely insane when you consider the eldar background.
Mind you, those guardians will move pretty fast across the board with their rules... but at 9 points a model, I expect to see a bit more combat capability then I'm seeing now. Giving your soldiers very short ranged guns and expecting them to "run as fast as possible and then shoot and hope the enemy is dead" seems cruel even for Russian WWII standards...
The combination of 4+ armor, 18" range and the new battlefocus/bladestorm weapons... now THAT will open some interesting options in firefights. But that'll be dire avengers. ;)