I have the Cobb pipes, along with their cooler. I like them a lot, as they eliminate the restristor on the one side. That being said, I'm sure the the power increase, if any, is negligible.
As for those claiming issues with flexing, I will completely disagree. I've had mine on for 5k miles with no issues whatsoever, and that's including WOT 3rd gear runs for my Protune. So this is not an issue whatsoever.
This is like a downpipe. Nothing proves any gains, but it absolutely can't hurt, and will benefit with upgrades......