I'm running -2 in the front & -1 1/2 in the rear and judging by tire wear I could easilly run betyween -3 to -4 up front for a larger contact patch BUT
my tires are smaller than yours.
You should give Lorin muller a PM at folcaljet , he runs 275's i think, and he's in FSP. he goes by omnifocus
he finished 3rd. in class at the solo2 nationals this year!!!