I really doubt the brass is worth it. I have the FRPP kit which includes the cap and rotor and the peice of plastic that goes from the cap to the distrubuter. It is pretty thick heavy plastic, which is good. Weather it is brass or aluminum, that will have nothing to do with crossfire or arcing, it is the insulation that prevents that. I replace mine once a year.
Beware of alot of these "performance parts" it's all marketing.