... and just how bleeding heavy that beast is with a 24-70L and flash after 4 hours.....
And the 24-70 f/2.8 is still going to be as heavy with the 5D, as are any of the longer, "fatter" lenses needed to get any high speed/low noise advantage out of using 35mm format instead of a smaller DSLR format. ("Fatter" meaning with larger front elements to provide the larger effective aperture diameters which are physically essential to get increased total light gathering speed and better high speed/low light performance, regardless of sensor size.)
If you have much need to use that lens wide open at the highest ISO with acceptable noise levels, you are stuck with that weight. Otherwise, you might well be better of with a smaller format DSLR with shorter, "slimmer", lighter zooms (e.g. 20D if Canon offers a fast enough EF-S standard zoom, D2x with 17-55 f/2.8, the expected new higher resolution Olympus E model with 14-35 f/2).
Remember that as soon as you stop down for the sake of adequate DOF, the low noise advantage of a larger sensor disappears: DX or 1.6x formats can then get the same DOF at a bit more than one stop faster, and so at about half the ISO needed with 35mm format; Four Thirds can be used about two stops faster and so at one quarter of the ISO speed.