I had one as a project bike a few years ago, never got to hear it run before I sold it but I've heard their monsters! Thankfully I have an XS1100 to restore
My last version of XS was 880ccm, increased compression to 12:1, bigger valves, machined channels for in- and outlet, modified exhaust system from the 850, two oil coolers, other carburrators and N2O injection. On the dyno I had 95 hp and 120 with injection.