I put some doubleshot ABS caps (pretty thin Taihao) on my QFR and you can't see the light through the Caps Lock or Win Lock keys. I put a couple on the Code and it was the same deal. Holding them up to the sun just now, no light shines through the top.
The thin Keycool PBTs let some light through but looked terrible.
Can't say for certain about WASD's normal caps...I could dig up the QFR's stock caps which are stupid thin and try them out.