That flight is about the same duration as EWR-SFO. Would you pay $550 and the miles to upgrade the transcon? I know I wouldn't - in fact I wouldn't even waste a GPU on this unless I had one going begging. Even though it's a red-eye, you'd do well to get more than 3 hours sleep - I'd sooner nap (or not) in coach, and then crash on arrival.
Different equation if you've got something important to do in Shannon in the morning, of course.
Ultimately it's opportunity cost. If you're cash and miles rich, then OC is minimal. If not, then it's large.