I don't need to tell you how important it is to have a wide open aperture when shooting video (on most occasions) - that's unless you want to have to crank the ISO up and get loads of noise and grain.
I've been searching for a zoom lens that is also fast (f/1.8 or 2) - but I can't find one. I can of course find prime lenses with a low f-number, but the same can't be said for a zoom lens. Even a 50-85mm or something would do. It needs to be a zoom lens to give me the flexibility to get different shots and control DOF etc.
At what price? The new L 24-70 is rated pretty dang highly by those using it. While only 2.8, I think it's likely to be very close to what you need. Have you actually tried shooting at 2.8 for your work? Might work out.
The other side of this issue is that you might change your camera to go with one with a higher native ISO. I think the C300 starts at 800, if my memory serves me well. That brings you into the ability to use 2.8 lenses quite nicely, which is what Canon and others normally have in a zoom. Not sure where the new C100 fits with it's native ISO.