After having the kit installed, I was running about 27.5 hwy on the way to Florida (1200 mi trip from NY) with the SS loaded like this and had 4 adults in the car. I'm at about 29.5 hwy by myself and heavyfooting the pedal (just can't be avoided with a kit equipped HHR).

Mowgli's right. Barton Birks was great by me re the service dept (sales dept is another story; they're shuysers). Mine's been installed for about 8 mos and 7k mi and hasn't missed a beat as of yet.