Levelling hand shot footage horizontally can be a time consuming task, especially if the levelling changes during the shot.
is any of you familiar with an FCPX plugin effect that will automatically level footage throughout the clip, based on e.g. an assigned horizon ?
Interesting thought. It would be possible through image analysis to have a plugin guess at a correct horizon adjustment. Some images would be dead simple - images where you can actually see a straight horizon. Otherwise it would have to look for various vertical/horizontal edges in the image and correct for those.
In the mean time, I recommend the FREE Dashwood Level Horizon plugin that is part of their Editor Essentials package in FxFactory. It automatically scales the image up to allow for the horizon shift and includes a handy grid overlay to assist in determining the proper horizon.