When you say 1 stop better, are you just talking about sensor noise performance or AF sensor performance?
My question is purely about sensor performance and why f2.8 will stay that important forever.
There is also another thing that you may not think of, is that lenses wide open generally don't perform their best.
... unless you're scraping together the money for a Canon 24-70L2
basically having a wide aperture available is like a condom
its better to have it and not need it than to need it and not have it
Ok, thanks for the answer from a real world pro, so you're on the side that even f2.8 isn't really "fast" enough and it just happens so that the fastest zooms available are f2.8. So if I understand you correctly you'd rather go with a Tamron f2.8 + a faster prime like the Sigma 35mm than a Canon 24-70/f2.8 for the price of both combined?
Well, even the Canon 24-70 v2 performs better stopped down. I never said that wide-open you can't get a lens that performs awesome. But if you stop down the Canon 24-70 v2 it gets even better.
That said, what lens(es) you chose often boils down to shooting styles, convenience (a high quality 24-70 is a lot easier to go 24, then 35, then 50 than changing primes), and can it get you what you want. Is it the best tool for the job, for what your budget can afford, and for how and what you are shooting.
Personally, while I wouldn't quite call myself a shooting pro, but I am shooting the residency for a small dance group, and while I'm renting the Tamron 24-70 for the next performance in a week, and probably buying that lens in the next 3-4 months. Not quite the weight of wickedwombat, or a couple of the other forum members, but I always seem to shoot in low or crazy lighting and I've found fast primes or f/2.8 zoom is practically a necessity. If you're shooting in decent lighting, or a studio or something, then ask if you really need a f/2.8 zoom. For me, I'm not shooting in those environments, so I've said yes, I do need a f/2.8 zoom (and the 70-200 some day, *sigh*).
Do you find yourself wide open all the time even as the ISO climbs higher and higher? Or are you comfortable going with a slower shutter and/or higher ISO and find an f/4 zoom is fine for you?