I've got the ODL bumper with PIAA 510 Crystal Ion Fogs in 4". They sit a little behind the opening in the bumper, so the plastic grills will likely clear. That said; I removed the grills on mine.
The grill cover white lettering showing though the opening, for example on the ARB and other custom plated bumpers looks cool. You would think the OL 4" opening would be universally fabbed to fit most applications, especially a popular brand like PIAA. I would content; the buyer of a $1000 bumper would purchase a brand name light as opposed to an off brand like harbor freight. Sitting behind the opening would appear to offer challenges to light disbustment.
Coincidently, when I bought the bumper I asked about what lights would fit. I was told it has a 4" opening. Further, it was suggested that I purchase a $15 a set of HF driving fog lights. I wish the bumper was as valued priced as Harbor Frieght......LOL :naughty: :jk:
Lastly, you mention without the cover the lamp sits behind the opening; how does that affect the projection of the light. It would seem, some of the light would spill on to the back of the bumper.