I used the Magnum brand cables and highly recommend them, great product. +8 on all the cables should work fine for you. My bars are basically 18" (16" Carlini Flying apes) and i went +12 but they were just a little long, could've gotten away with +10.
Magnum cables are excellent but somebody tell me what is wrong with Drag Specialties. I was a +10 on just the clutch and the rest were 6 or so. I think I was longer on the clutch because of the pullback on the apes as well. I'm sure you'll love em when they're on.