Only the really bright stars show any aberrations. Using the lens at F4 would tighten up the brighter stars but halve the speed. Heres a high resolution picture of Carina taken with an 80ED @ F7.5 and the 5DH. Field curvature among others is a problem here.
1.6M
FOV 3.438X2.292 deg.
http://users.bigpond.net.au/avandonk/CAR01ac.jpg
Compare to the same object taken with 300mm @F2.8
1.9M
FOV 6.876X4.584 deg.
http://users.bigpond.net.au/avandonk/Car_16_1.jpg
The slight aberrations in this image is not even visible in a lower resolution picture.
I have also shown low res images, the third is the FOV of the 20D with the 80ED 2.292X1.433 deg.
Bert