Quality wise you will loose almost nothing. I've attached a chart which is just a side by side of the Sigma 24-70 EX against the Canon 17-55 IS USM. The data is the photozone.de data, all I did was put it in a graph.
Sigma tested at 24mm/40mm/70mm
Canon tested a 17mm/35mm/55mm
So roughly comparable.
You loose IS, but you gain better high ISO performance so you can keep the shutter speed, if you are shooting static objects or people in dark rooms and want to keep the ISO down, bounce your flash of the ceiling or a wall or some kind of diffuser. I'd argue this will give you better results than the non-flash IS shots.
Landscapes, seriously why buy a 5dII and 24-xx L lens without already having a tripod. But for arguments sake lets say you want IS for landscaps, its usually daylight you shoot your landscapes in which case you will have shutter to spare, and even can go to ISO400 without seeing any noise, if you needed too. Remember the key point is your FF camera will handle the noise a lot better.
They way I see it you will end up with almost exactly the same results, just using slightly different gear (and relying on ISO performance and mono/tri-pods instead of IS). So I don't think you will loose anything, but I don't think you will gain anything at the same time. If it ain't broke don't fix it.
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