Why root-prune when you repot?
I have spent literally hundreds of hours digging around in root-balls of temperate deciduous trees collected from the wild, nursery stock bonsai candidates, and trees that have already had some root work in preparation for bonsai training. The collected trees are a challenge, usually for their lack of roots, and have stories of their own. The nursery stock is probably the closest examples to what most of your trees are like below the soil line, so I'll offer my thoughts for you to consider or discard as you find fitting.
I've purchased many trees from nurseries that have been containerized for long periods. Our bonsai club, just this summer, invited a visiting artist to conduct a workshop on Mugo pines. The nursery (a huge operation) where we have our meetings happened to have purchased several thousand of the Mugos somewhere around 10 - 12 years ago and had been potted-up into continually larger containers since. Why relate these uninteresting snippets? In the cases of material that has been progressively potted-up, large perennial roots occupied nearly the entire volume of the container, plant vitality was in decline, and soil in the original root-ball had become so hard that in some cases a chisel was required to remove it.
In plants that are potted-up, rootage becomes entangled. As root diameters increase, portions of roots constrict flow of water and nutrients through other roots, much the same as in the case of girdling or encircling roots on trees grown in-ground. The ratio of fine, feeder roots to more lignified and perennial roots becomes skewed to favor the larger, and practically speaking, useless roots. Initial symptoms of poor root conditions are progressive diminishing of branch extension and lessened vitality. As rootage becomes continually compressed and restricted, branch extension stops and individual branches die as water/nutrient translocation is further compromised. Foliage quality may not indicate the tree is struggling until the condition is severe, but if you observe your trees carefully, you will find them increasingly unable to cope with stressful conditions - too much/little water, heat, sun, etc. Trees that are operating under conditions of stress that has progressed to strain, will usually be diagnosed in the end as suffering from attack by insects or other bio-agents while the underlying cause goes unnoticed.
Potting-up temporarily offers room for fine rootage to do the necessary work of water/nutrient uptake, but these roots also soon lignify while rootage in the old root mass continues to grow and become increasingly restricted. The larger and larger containers required for potting-up & the difficulty in handling them also makes us increasingly reluctant to undertake even potting-up.
I haven't yet mentioned that the dissimilar characteristics of the old soil as compared to the new soil when potting-up are also a recipe for trouble. With a compacted soil in the old roots and a fresh batch of soil surrounding the roots of a freshly potted-up tree, it's impossible to establish a watering regimen that doesn't keep the differing soils either too wet or too dry, both conditions occurring concurrently being the rule rather than the exception.
Most who read this would have great difficulty showing me a containerized tree that's more than 10 years old and vigorous, that hasn't been root-pruned at repotting time (Trees in extremely large containers excepted. Growing in very large containers is similar to growing in situ). I can show you hundreds of trees 20 years to 200 years old and older that are in perfect health. All have been root-pruned and given a fresh footing in in new soil at regular and frequent intervals.
Acers are one of the most forgiving of trees when it comes to root pruning. The process is quite simple and the long term benefits include best opportunities for plants to grow at or near their potential genetic vigor, and stronger plants that are able to resist the day to day perils that bring down weaker plants. Root-pruning is a procedure that might be considered borrowed from bonsai culture, but bonsai culture is nothing more than refined container culture and to restrict the practice of root-pruning to bonsai only, is an injustice to those of us who simply enjoy growing trees in containers.
I didn't mean for this to be so long. I haven't even touched on the methodology of the process yet. If I've presented my case adequately and there is interest, I'll follow with the short version of how to root-prune & repot. If not - I had a good time writing this anyway. It gets really boring around most of the forums at this time of year. ;o)