Jacobi - I just transplated 6 apple, peach and pear trees about a month ago that had been in the ground for about 1 1/2 years, as I just created my "orchard" space in my yard. I severely shorted each of their rooting systems, I'm quite sure.
Everyone of them is doing grand, two are flowering, another couple sprouting growth - me thinks that Dormant trees easily handle severe root pruning - you might want to cut some of the branches off the tree as the smaller root structure might not be able to feed the full canopy that is there now. But I think you should do it - perhaps add a root stimulator or some kelp powder (made in to a tea) to help with the root growth...
I remember when I got my dormant 15 gallon 4 in 1 low chill pear and when I transplanted it from the nursery pot, the root ball fell apart and all there was, was just this stalk of a thick root, chopped off at the bottom (in order to fit in the pot) with a couple swizzles of small roots coming out its side with nothing else and it recovered and grew - no problem.....
FangoMang