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.
5Dsr, 7DII, G5X, α7R II. Zeiss ZE 21, 100. Canon EF 15, 11-24L, 16-35 F4L IS, 24-70 F2.8L II, 24 TS-E II, 100L macro, 200 F1.8L, 70-300L, 70-200 F4L IS, 70-200 F2.8L II, 100-400 II, 500 F4L, 1.4x III, 15-85. Sigma 8-16, 35+50 ART, 85, 180 F2.8 OS, Sony: 55 F1.8, 135 STF. SY 14, 135.