In my mind the only reason to deviate from the stock plugs would vary greatly on what modifications where done to the vehicle and the end use of the vehicle IE road racing predominately or street use ECT.
I have a whipple and headers, I'm running 15Lbs. and had a misfire code and a bad misfire. I checked the plugs, they all looked good. I replaced them with NGK TR7IX iridium plugs and never had the problem again.