Now for the mahonia, a much easier case. Have courage here, it’s a bit late to prune growth you hope will flower in the very early spring, or even autumn, but to be honest, I have rarely seen a mahonia look the worse for pruning, even when flowering time comes. Inexplicable. They just seem to like it. So simply cut at the height where you would like the next set of fans of leaves to emerge. It’s usually possible to hide the cut ends amongst the existing foliage. If you want growth from low down, cut right to the base. In general mahonias make all their leafy and flowering growth at about the same level on their ever-lengthening stems. It’s up to you to distribute it up and down the structure, by varied cutting.
People get strong feelings about mahonias, I take them with equanimity and a measured appreciation for what they have to offer (almost tropical looking leaves, drama, architecture, flowers in winter, shade-loving etc). I do feel a stir of affection for mahonia japonica however, with the lemony flowers that look like tiny daffodils and smell of lily of the valley. But less drama and VERY dark leaves.
Would I plant any of these shrubs now? They're redolent of 80s and 90s planting and you're unlikely to find them in the Garden Design Journal email@example.com amongst the wildflower meadows, the land forms and the Cor-ten. But I would plant the viburnum for the winter flowers - they screen by attracting and halting the gaze in the winter and their tall leafiness in summer can effectively background your massed grasses if you can get the sun to shine from the right direction. The mahonia looks worst when mixed with complicated colourful borders, it needs simplicity and other greens.