The 24-70 f2.8L is a fine lens, and one I use in the studio a lot of the time. The 28-70 f2.8L is also a fine lens, and the predecessor of the 24-70 - many owners of the 28-70 have seen no need to upgrade to the 24-70. Neither lens is all that easy to find second-hand because their owners seem to hang onto them
They also seem to hold their value well (you'd expect the 28-70 to have dropped in price after the 24-70 came out). Either of these would be a nice upgrade and would give you a brighter viewfinder with the f2.8 maximum aperture.
I haven't heard of a 28-80mm - have you tried googling it? Are you certain it's an EF lens? I think there was an FD 28-80, but that wouldn't fit on an EOS body.
If you can, I'd try the lens on your body and see how much difference it makes. Paraphrasing what RFS says, it may not have any effect on your ability to get the image you want.
EDIT: Wikipedia says that Canon released a 28-80mm f2.8-4.0L lens in 1989 - sounds like it's an old design, but not necessarily a bad one - at least it is USM.