It looks to me like the limiting factor regarding max speed/flow is really the heating chamber. That seems to be identical for all Bambu nozzles which is why .6 and .8 do not translate to higher speeds.
High flow nozzles have a larger internal surface area in the heating chamber. At least for the ObXidian, the next limiting factor upwards is the power output of the heater, or more precisely the boards power capability which prevents safely installing high power heaters in the hot end.
Getting the .2 and .6 nozzles can still be a good idea. .2 for fine detail, .6 for prints with transparency or reinforced material.