Regarding Kibblewhite bronze guides, they use C63000 as far as I know, it is not the best available bronze alloy for valve guides, it’s good, but if using stainless valves it’s better if they are nitrided, they also require more clearance than normal, a bronze alloy containing about 3% manganese allows for tighter valve to guide tolerance and will last considerably longer, but I’ve never seen XS650 guides made from it, you’d have to make them or have them made,
one of the main benefits of bronze particularly for exhaust valves, is heat dissipation, but if you have to run greater clearances a lot of those benefits are lost.