It is not true that you can assume same shutter-speed by comparing F-stops only. The have to have same focal length.
F2.8 on a 10.5mm fish-eye is a different story than F2.8 on a 70-200mm lens, for example.
Also, not every lens has the same transmissivity. For example - many lens review note varying degrees of corner darkening or "vignetting" - but the concept applies to the center of the lens as well.
This latter part has somewhat less practical implications, but the focal length does have a significant impact on the resulting shutter speed.
This may all be just a tangent on my part, since drax0r probably implicitly meant to compare two lenses at the same focal lengths.