There are a few boutique laptop builders out there. They offer a little more customization then the standard OEMs, but it isn't worth it. You will end up paying thousands for minor things. Sager, Clevo, Falcon Northwest, Xoticpc.
Best bet is to look at the standard OEMs, they'll have performance laptops that you can customize with screens, cpus, video cards, drives, and memory. You are pretty much stuck with their form factors though, as all of this stuff isn't that modular when you cram them under an inch thick.