I think this lens would be amazing, but i think it needs to be Â£400 cheaper.
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If the 5d mkiii had noise improvement would it make using f2.8 better for low light as you can use higher ISO?
Yes, but at some point there's a limit. There are two parts of noise, the camera's own ("read noise") and noise in light itself (photon shot noise). Most people forget about shot noise and assume that noise at high ISO is just contributed from the camera electronics. Actually, the shot noise is often dominating (I have not found out where the line goes, but perhaps some other reader of this forum knows).
The signal-to-noise ratio of the shot noise reduces the more light you gather, that is lower shutter speeds, larger apertures, larger sensor size. That is you always want to gather as much light as possible, to be able to use as low ISO as possible.
One other factor to know about is that not all f2.8 are the same concerning light transmission, and it is not only about vignetting. There can be quite large differences between lenses, some lenses can transmit a half stop less light than others at the same aperture. For example the 24-70 f/2.8 has a transmission corresponding to ~f/3.4 while a large aperture prime lens set at f/2.8 typically has much better transmission (did not find an example measurement unfortunately, but it can easily be observed in testing).