I did a bit more playing around with Firepower today, put LEDs into the back box.  In looks pretty good, you have to look reasonably hard to pick out that they are LEDs. The back glass is pretty dark so maybe that helps?  In the pictures you can see the LEDs shinning through, but in the flesh they don't show up with the same definition of the bright spots as in the picture.

I also put a few in the slingshots and once again they seem to just blend in, not as stand out as in the pictures. I'll leave them all in.