I also have read the reviews. Yesterday I got the Canon 24-70mm II and the Tamron 24-70mm VC at once to be tested against my Sigma 24-70mm HSM:
The Sigma is the worst, particularly at 70mm. The Tamron also is unuseable below F8 at 70mm, IMHO. I would estimate only two F-stops efficiency for the stabilizer of the Tamron at 70mm. May be more with shorter FL, but I haven't tested this. The TDP test results of the Tamron at 70mm is exactly what I discovered with my sample.
Unfortunately, the long focal end of the Canon is comparably short, I guess 62mm.
In summary, I am overwhelmed by the IQ of the Canon, based on the reviews I have not expected this. The difference between the Tamron and Canon is much more than the PZ test implies.
5DII, 5DIII, 550D, Nikon: 14-24, Zeiss ZE: 21, 50+100 macros, Canon: EF 16-35 II, 24-70 F2.8L II, 24 TS-E II, 70-300L, 70-200 F4L IS, 70-200 F2.8L II, 400 5.6L, 15-85, Sigma: 8-16, 35+50+85 F1.4, 50 F2.8 M - Contax 35 PC shift, Leica MR 500 F8, Tokina 12-24, leitaxed Contax, Olympus, Leica lenses