My experience is in part that my best zoom lenses performed as well as mine primes, but I generally tried to use high end zooms and not high end primes. One issue to consider is which aperture you shoot. High end primes are often built to handle large apertures well, but going down to f/5.6 or f/8 is a great equalizer.
There is a also a lot of sample variation. The Canon 24-70/2.8 has a problem with some plastic part (bushings on the fron lens group) wearing down. The lenses having this problem are said to be awful. The problem can be fixed by Canon.
Zooms often vary in quality depending on focal length.
I have a Sony 24-70/2.8 ZA (which is called a Zeiss lens). In general it's quite OK, but corners are horrible at f/2.8, you need to stop down to f/8 for really good corners. On the other hand the lens has a large sweet spot at all apertures, and in the sweet spot that lens is probably pretty good.
One issue to consider is that top performance is available within a very short DoF, I recently tested a Sonnar 150/4 for Hasselblad, and two zooms an older Minolta 80-200/2.8APO I have and a Sony 70-400/4-5.6. These tests were made on APS-C and would correspond to 54MP on full frame. (Yeah, I also made tests on full frame).
Here are some actual pixel crops:http://echophoto.dnsalias.net/ekr/Images/Zeissness_3/F8/flower.htmlhttp://echophoto.dnsalias.net/ekr/index.php/photoarticles/73-sonnar-150-cb-on-dslr-using-arax-tilt-adapter?start=1http://echophoto.dnsalias.net/ekr/index.php/photoarticles/73-sonnar-150-cb-on-dslr-using-arax-tilt-adapter?start=4
Interestingly, it seems that most readers preferred the Sonnar 150/4 but the differences are very small. There were discussions about the color of the orange, BTW, but I measured the actual orange with my Color Munki spectrometer and it reproducs essentially exactly.
MTF plots indicated a very tiny advantage for the Sonnar in this case, but the curves are very close. http://echophoto.dnsalias.net/ekr/index.php/photoarticles/73-sonnar-150-cb-on-dslr-using-arax-tilt-adapter
These tests were made at 3.9m object distance at f/8. I would guess that the zone of optimal sharpness is perhaps +/- 5cm.
Than of course, focus is by Live View at 11X magnification. Self timer and studio flash was used.
Interestingly, although the Sonnar has a long focusing thrust, exact focusing with live view is less than 0.5 mm back and forth on the focusing ring. You can barely move it without loosing some sharpness. The zooms are not easier to focus.
The test above was in the sweet spot for all lenses.
Several zooms have been reported to be equal or better than the correspondent prime lenses. Among which I've read include Leica R 35-70m f2.8, Leica R 70-80mm, Zeiss 35-70mm, and Contax 645 45-90mm.
I've never seen the two Leica R. They are extremely expensive now. I have the Contax 645 45-90mm. It is a very sweet lens. It makes my 80mm and 45mm lens sitting on the cold bench almost forever.
I'm cleaning up all medium format gears. They are too heavy. I'm switching to light weight 35mm format. I'm in search of the zoom lens that can give me reasonably satisfaction like the Contax 645 45-90mm. It doesn't have to match the 45-90mm -- 35mm format to match the Zeiss medium format? yes, I know, it's impossible. But I'd like the best possible.
Any other zoom that fall in this category? I've compared Canon L 24-70mm 2.8, good but not yet, it has to be better than that.