5/16" is stock size. -6 is 6/16", what's a 16th of an inch going to change?
stock lines or -6 is good to 600 hp, -8 is good to 1000.
to add to wop's comment - the biggest restrictions are the regulator itself, and the factory banjo bolt coming off of the fuel filter. that banjo fitting really restricts flow, big time. if you'd like to correct that bottleneck, either run an adapter to utilize -6 from the factory fuel filter, or add an aftermarket fuel filter and go -6 from that to the rail.
edit: you have a -8 feed already, that's what i get for trying to read too fast /ubbthreads/images/graemlins/rofl.gif