You shouldn't have any problems with the axels or tranny on the street with street tires. You will run into problems with those if you get sticky tires and suspention that will let you really hook up. And in that case, you can break things with way less than a KB.
I have absolutely zero issues with the 9lb, and run premium 92 fuel. If it is a money issue, get the 6lb. But if you are not getting the 9lb because you are concerned about the drivetrain, that should not be a concern. The 9lb kit
is the way to go on the street.
As for the KB air kit, most of the increase comes from haveing the larger MAF and TB. I used the MAC CAI, Lightning MAF and Accufab TB as I think the KB kit looks a little cheesy for my tastes. I have seen guys argue that the KB air kit is supposed to work better, but my numbers are right in there with the guys with the KB kit and tune, so I don't know if that is just marketing hype to sell parts or not.