The Obxidians are pretty cool in my experience. Just increase flow by 50%, slice and print. No drop in quality whatsoever. At least for PLA. Not sure how increased flow would interact with PETG’s poor overhang performance which is of course cooling driven, so not much the hotend can do about it.
However, the effect is probably even bigger for the .6 nozzle. The Bambu .6 is flow limited to the same limit as the .4, so not much use in going faster. The .6 ObXidian has more room to bring actual improvements in that regard. Only a max flow test will give hard numbers though (with which I did not bother) as @movingimage noted.
As for why E3D rather than Bambu, well, it looks like E3D has more nozzle experience than Bambu so, not really a mystery.
The only downside is that their expense means that it hurts much more when a nozzle gets bent …