We'd suggest you look at our Firemaster letterplate. It has been acoustically tested in a typical FD30 door leaf, with minimal impact on the acoustic performance of the door.
It could be that you have your eye too close to the glass. The benefit of the wide angle viewer is that you can stand some way back from the glass and still see a clear image (which is much safer) - but if you stand too close the image will be fuzzy. Try standing 8-10” back from the glass.