ok so I'm starting to think I need to get some new ignition wires, so I was looking at 8.5mm magnecors then saw that the 10mm wires were only 5 dollars more. so I was wondering if it would be better/worse to use 10mm instead of 8.5mm wires on a non-turbo 91 rx7... or does it even matter

bigger = better with wires, less resistance. Go with the 10.