If a plugin is applied to an FCP layer that is smaller than the final output, the plugin will still only affect the area within that layer's size, and sometimes appear to be cut off. To extend the effect beyond the edges, just make a nest from the layer and apply the plugin to the nest.
One way to make a nest is to create a new sequence, drag your footage into it, and then drag this new sequence into your Main sequence to nest it. Then apply the plugin to the nested sequence, and it should affect the area outside the original layer.