ok the iMovie thing I understand, as it may be limiting you to 1080p exports.
However, Shadowplay does allow you to export at 4k/60FPS despite playing at 1440p. My kid plays at 1080p/144Hz on a GTX 1080 Ti, but exports clips at 4k/60FPS/100Mbps bit rate.
In the Shadowplay settings menu, scroll...