Almost all lenses (except perhaps those costing k$) will have coma at full aperture in corners.. It appears you also have a problem with co-axiality...
I am using modified Canon SSC FD f1.4 50mm lens. It appears to be much better than yours, even at full aperture (1.4). I was trying other lenses (for Zenit and Praktica) , but so far this one is the best in this respect (coma at corners).
See here:
http://www.iceinspace.com.au/forum/s...ad.php?t=39293
But I had problem with uneven focus as well.
I solved this with one layer of copper tape (usually used for RF shielding) placed appropriately on one side of the lens seat (to tilt it a bit) , so the lens optical axis is now aligned with the centre of the sensor frame, and I have the good focus across the whole frame (lens focal plane is parallel to sensor plane.. still a bit of tradeoff must be accepted, though.. however this is for pixel-peepers :-) ).