I hear where you are coming from, but to be honest with you the 24-70 2.8 is one of the sharpest short zooms Canon produce. Firstly, if you want pin sharp images edge to edge you need to understand how to use this lens. Whilst the 5d is a superb image making machine it can have it's draw backs if it isn't set up right and I suggest you follow Jim Doty's advice about menu settings here:
http://jimdoty.com/Digital/5d_menu_s..._settings.html I used his settings for nearly five years with my 5d and never had a moments trouble.
Lenses on full frame will show up every anomaly. Focussing correctly to achieve pin sharp stuff starts with using the optimum aperture for nearly all your work flow unless you need to change it for effects. The optimum working aperture for the 24-70 2.8L is 5.6.
Sharpening: you need to get into a routine that will benefit your sharpening and not destroy by pixilation.
For some years now I have been using the Lab Colour method and you can find a good tutorial on how to do this here:
http://photo.net/learn/digital-photo...-in-lab-color/
In thirty years of photography I have noticed that much of the problems associated with focus is down to user error. Don't dismiss this as you will end up being dissillusioned with gear that requires some learning about and handling experience. I hope you are satisfied in your quest for perfection, though I doubt you will be.
I have just sold the 24-105 which I used for a good few years. That lens will give you the same problems if you don't shoot at optimum - f8. I own the 70-200 f4 L which is stonkingly sharp and worth every penny. The other lens I own is the 17-40, also sharp and massively wide on full frame.