The series 'totem pole' arrangement of caps is purely for the life of the caps, protection from over voltage. There is no sonic advantage. Caps today are not rated for higher voltages, or they get $$$ real fast as voltage rating goes up. Compare 2ea 350v caps (double the capacitance, as you observed) to 1ea 600v cap. Choose the method that makes the most economic sense. Do you need a cap to handle voltage that might peak beyond 600vdc? I don't think you'll have an option then.
But, yeah, two 450v caps for 900v? My guess is that's what they stocked, so that's what they designed around.