15.45 and 16.48 are suspicious numbers. It seems Canon want to be able to round down and sell them as 15mm and 16mm. I always hate cheating like that. I am not a lens engenieer, but if you have a formula for a 15.45mm lens, it should be technically possible to tweak some parameters and make it exactly 15mm. The same is true at the long end of tele lenses, where Canon often rounds up.
It is good though that Canon brings slower versions of its very expensive lenses with f/2.8. With a powerful IBIS you do not need much light and I usually chose an aperture just below the diffraction limit of the camera to get as much depth of field as possible. That's f/11 at my 18 megapixel camera. So f/2.8 for me is just something that makes a lens twice as heavy and twice as expensive and I only need it for low light hand held shots.