Thread re-up 
First just an information: as you can see some ktx are now available directly from the playground! (playground texture doc page will be update soon) + an addition of a cubemap dedicated to debugging or understanding how axys works. All ktx have been generated from the tools quoted in the doc.
Here an example which works perfectly for both Standard & PBR materials: https://www.babylonjs-playground.com/#86DHVJ#1
Now here comes the issue 
Are cubeTexture supposed to be available in ktx format? I think so but it seems not working:
- regular cubemap jpg files: https://www.babylonjs-playground.com/#UU7RQ#326
- ktx version, showing no errors but stucked on “Loading assets… Please wait”: https://www.babylonjs-playground.com/#UU7RQ#327