I am sure there are smaller brakes out there that are better than the OEM brakes.
On my -77 European model, the twin discs with the single piston calipers were close to worthless.. The older models with the 2-piston calipers and 2-piece single disc may be better. My old RD has that type of brake setup, and stops twice as hard as my XS. Obviously, the RD is a lot lighter, but still...
Also, I believe the stock XS discs are of a low friction stainless type of alloy, rather unsuitable for the purpose.
A cast iron or carbon steeldisc, combined with EBC HH pads, you will givea lot better bite and feel in your brakes, especially combined with an optimal size master cylinder and braided hose(s)
To me, it is also unclear what you want to achieve, is it a different look, or lighter weight, or both?
There are almost endless options, from twin 320 mm discs with 4 or 6-piston calipers, this would still be vastly lighter thans stock twin discs, and maybe 300% more brake power. At the other end of the spectrum, some Swedish builders have made tiny discs concealed as a small (5-6" OD) drums, with the caliper sort of integrated in the "brake plate".
Whatever you decide to go for, stay away from lazer cut stainless discs, instead go for cast iron Brembo, ISR, PVM, Beringer, BrakeTech, or other quality brands. I would stay away from a Chinese made brake as if it was Ebola!