All of the current available LT headers seem to be good quality. I'd stick with ceramic coated headers since these cars get hot under the hood. HP gains vary according to mods. A car with a blower will see more gains than a N/A car. On a stock car, I think it's around 10-15 hp depending on who tested them. I'm sure somebody will get on here and quote a recent article on the subject.
You will hear from lots of people, I would stick with the major brands like JBA, kooks, etc. I went with the JBA LT's and they work great. I have not made it to a dyno yet, but I deffently feel an increase in power and response. When I installed mine I also did the C&L Intake with a SCT Tune from Brenspeed.