I've never installed cabinets in a house that wasn't a bit out of square. Trying to fix the drywall never occurred to me. Maybe it's because I'd much rather make sawdust than tape drywall. I need to fix a "professionals" job in my room over right now. I could call the builder back and they would fix it but they would also make a huge mess like they did the first time. I have the flooring in now and my wife would be extremely upset when she saw what I am sure they will leave behind. It is the peak of a cathedral ceiling. When I did another room I used the setting compound to create a smooth radius and it worked pretty well. That is the thickest I've spread mud and it worked fine. But it is also up in the air where it won't get contacted. I'd be worried about thick mud anywhere it could get bumped.