Im pretty sure people only run it when using s13 struts? GTR brakes bolt on with gtr hubs and discs i think? Often hear of GTR brakes on s13's so i think thats the easiest way to go about it.

But then you'll need to run 16's, don't think 15's would clear. The fact that your rims are neg offset has nothing to do with how much brake clearance they will have, that depends on the spoke design or concave type.

For example meshies, ssr mk 1,2,3 are shit for clearing brakes, as they have a flat back surface where as watanbes, longchamps, are better due to the spoke curve.