24-70L is notorious for sharpness issues at the wider ender, as well as AF problems. Some of the pros I spoke to in UK also told me the same problem and somehow images aren't just as crisp.
If 24-70 is a different lens recipe altogether or improved like those new Nikon glass, I really won't mind getting it since it's for work. Other than that, I really don't see how IS is going to help, seriously.