tom, if the people that are making those headers are using 8 ga. tubing, i believe thats what you said, then thats the reason he can get that clean colorful look. i've welded a lot of stainless over the years, most of it on guys toys ,and looks meant everything to them. anyway, between the the thick walled tube and those hefty flanges hes using, its pretty easy to get what you are looking for, because the material is not overheating, overheating is what is causing your problem.
If you have to use 16 ga. then like someone mentioned, try pulsing, either with the pulse on the machine or manually with your pedal or finger control. i use a finger slider amptrol, it works well for me. also, move as fast as you can, and still weld good. try cooling your part prior to welding, i used dry ice before, it worked well, and kept distortion down also
i hope this helps, mike