Op (Alexandros), are you kidding me?
No, you are not the only one. I guess you haven't been following the dynamic range debate lately?
Canon is known for banding (pattern noise) and now also underwhelming dynamic range. The 5D line has always had banding, and it seems that Canon doesn't know how to fix it, nor how to increase low ISO DR (at least not during the last 5 years).
Nikon on the other hand, are leaping forward with the sensor in their D800. Well, they are getting their sensors from Sony, but that doesn't really matter.
Canon needs to step up.
Please post your images showing the issue. Show your D800 images as well. I somehow am skeptical, having both bodies.
Here is one from my 5D MK II, notice the severe banding at the left --- wait, thats just a window shade.I at ISO 51200
And from my D800 at ISO 6400. Both have a ton of NR. Very grainy, and not much more detail that the 5D MK III at ISO 51200.
He is talking about LOW iso only. Nobody said the 5D3 DR is worse at 6400 or 51200 or whatever.