Lens repair

Hi
I was hoping that some of you could help me with a problem.
I have had an accident with my brand new (only one month old) canon 70-200 2,8L IS USM. It had a quite ugly drop and afterwards i could not evet turn the zomm ring. I have now had it to repair at the local Canon service senter, who said that they were able to repair it. They changed front ring unit, zoom unit and the EMD/IS. I have had my doubts about if they would be able fix it witout compromising the image qality.
Today i have tested the lens as i got it back, with a mirror and a pice of fine squared paper and it seems verry soft in the cornors. (And the softness seems uneven).
Now i would like to ask you if you know, if it is possible to fix a lens as new after a drop like that. Ore if it is impossible without compomising image quality.

thanks in advance

Christian

(sorry if my english is bad)