(Excuse the slight necro-post.)
One thing that I've always been interested in is the concept of T-stops, in addition to the usual F-stops. For equal F-stops, two lenses on the same camera will have the same DOF, but not necessarily the same brightness. This is where T-stops ('T' for transmission) come into play. Differences have typically been small for lenses of the same type (zoom or prime) and aperture, so usually just F-stops are given.
However, in looking at the 24-70mm f/4L IS, I did happen to look at DxOMark's (I try not to focus on curves and such, but I don't discount them completely, either) transmission rating for the lens. It actually had a T rating of 4.0. This is not uncommon for a prime, but unusual for a zoom. Comparing with the 24-105mm, which is rated a 5.1, I found that it is approximately 2/3 of a T-stop brighter across the range of focal lengths. This may not be huge, but it might be just enough to keep motion blur to an acceptable level in lower-light situations or in fast-action situations by using a faster shutter speed or to allow for less noise by using lower ISO.
Almost as surprising is that it is just 1/3 of T-stop behind the 24-70mm f/2.8L (first version), which is rated a 3.6! Conversely, the 24-70mm f/2.8L II is rated a true 2.8. These relative differences are almost certainly due to the newer designs being more efficient and/or having better coatings.
Unfortunately, that doesn't do much for the Tamron 24-70mm f/2.8 VR, as that lens is also rated an f/2.8!