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I'd prune it hard down to about a foot from the ground and then mulch it. Mahonia can take a good hard prune. Mahonia prefers a shaded or semi shaded position so a move will help as well, being in full sun won't help. Keep the roots moist and it may recover.
Agree with Dave - most Mahonia prefer shade and in full sun look quite ugly, exactly as you describe. Cutting it down should revive it. Place it in shade this time and use the sunny spot for other plants.