I am thinking of getting a rear disc custom made to fit the gixxer rim with the same dimensions as the FJ rear disc. (an FJ rear disc to fit gixxer rim) so I can use the original FJ rear calipers. Would make it simpler. I don't want an underslung caliper and rotating the gixxer so that it is above will have the bracket looking all wrong. I tried the fit and it seems to line up the same (disc through caliper), if not precise it's only spacing that would need correcting.
I can get it done through "MetalGear International Pty Ltd" based in Queensland. The disc I have is MetalGear.