I've looked at graphs like this many times over the past few years:
One of the things that I notice in that chart is that countries that are reputed to have strong union movements have market inequality outcomes that don't appear to be all that much better than those with weaker union movements.
So this post offers some cross-country evidence about the empirical link between unionisation and inequality. All data are taken from the OECD. The inequality measure is the Gini coefficient, which means that this post has nothing to say about 'top end' income concentration. (See here for why I think the distinction is important.)