I sure that could be one factor. But it is a big ask for any lens. Canon tries to overcome this by having a microlens on each sensor element to concentrate the light.
http://web.canon.jp/Imaging/cmos/tec...gathering.html
I suspect even the 300mm F2.8 L will have some vignetting. with astrophotography we are really pushing everything to the limit.
I will have to improve my image processing methods as CometGuy and Itchy are far more knowledgable than me.
Bert