The 15-30mm is a 3.5 - 4.5 as well. The full aperture shouldn't affect the brightness of the final image, only the viewfinder image. I have always had to add 1 - 1.5 stops with the Nikon lens because it generally gives darker images than I like and is at its best in bright situations. The 15 - 30mm is a DG lens so it is designed to cover a full frame and would deliver as much light to the sensor as the Nikon. I agree that the wide angle lens collects light from a wider area but the result on the final image should be the same. If that was not the case telephotos would be very very dim whilst wideangles would be ever so bright !
Colin.