Never heard of a FO getting 80-90% UG's so if you're happy, stay there. But for virtually everyone else, the difference between FO and GM is huge. Can't imagine why anyone would want to hold at FO, just look at the difference in benefits...
https://www.delta.com/skymiles/about...ance/index.jsp
And those are just the published benefits. With IROPS and op-ups, you may get more.