I have had professors that had me write huge essays and then proceeded to just put a huge ass check and never really read past the first paragraph. If I plagiarized and cited my frustration with all those other professors just glossing over hours and hours of my work, would you accept that excuse?
I guess what I am tryin to say is that your 80 students who did not cheat have as much right to be pissed at you as your right to be pissed at the 28 students that did cheat.
As for the evals, I always found them to be kind of a joke and would agree that the better grades you assigned, better your ratings would be. As for me, I was too lazy to bubble in stuff.