Hi there,
Typically, it depends. Generally Arce Palmatum are not very fond of being root pruned, but if in good health they can be very tolerant to this technique. To be in the safe side I´d say no more than a 50% root pruning.
Now taking it into the real world... I have a Palmatum myself that more than a month ago I transplanted it from a training pot into his first relatively small "Bonsai pot". In order to make it fit into such pot I trimmed approximately 75% of the root mass. And now there it is... It has sprouted all over the place and it seems as vigorous as it always has been.
The only concern that is still puzzling me is when the hot summer arrives. Usually with this tree I only had a couple of leaves scorched due to the high heat of this time of the year... That good it was its root system. Now, I´m not sure that at least for this growing season I won´t have leaves scorching all over the place. I´m keeping my fingers crossed.
Alain.