I'd say a professional UI designer is capable of producing objectively good results. He'd first sit experienced photographers and graphic designers in front of GIMP and identify their biggest gripes with the UI and then would develop a hierarchy of measures to fix them. And test the results.
Honestly, they'd probably have to spin it off into a separate, UI-focused project that starts with minimal features and gradually adds more every time a good UI solution is found. In the order of what real-world users identify as the most needed features.
In other words, I believe what GIMP needs is a partnership with an idealistic professional workplace willing to use it as an experiment and communicate their day to day issues.