8000k aren't as bright as 6000k in terms of usable light.
ZZP sells kits, eBay sells kits, tons of private vendors sell kits... They're all more or less the same once you get them out of the package. I bought mine from a vendor www.higherlevelauto.com . He's a sponsoring vendor of NAIOA, and he offered me a military discount.
I don't know about GPs, but on Impalas and Montes you run the risk of damaging your stock wiring or blowing fuses if you don't run a relay harness. HIDs draw a lot more power than halogens during the initial warm up phase. After that, the power draw drops to below that of halogens yet still retain more light output.
