I just upgraded to 10.1 and everything inside FCP X looks way too dark.
it starts in import preview viewer and looks the same in clip preview and timeline
as well. Export to original media is again as it should be.
It the same on Apple's standard display profile and also on my own cailbrated (D65 / sRGB / cd120)
it was perfect in OSX 10.8.5 and FCP 10.0.9