Mine are always rootbound when I buy them. Haven't had much problems I could really attribute to the rootbounding, though. Sometimes they have really been in the pot too long, and the plants look somewhat stunted, maybe a few yellowing lower leaves, leaves kind of leathery instead of young and full of growth, etc., and these don't seem to ever amount to anything--figure they are too old, which is likely on a clearance sale, but also figure they probably didn't get good care and dried out pretty badly a few times. However, I have been told to take part of the rootball in each hand and pull sidways until it has split about halfway up. This breaks the roots and tends to stimulate new root growth. In the ones that you dug up, were there any new roots extending out from the rootball? Have they been getting enough water? Transplants dry out surprisingly fast before the roots get extended out into the soil, especially if the top of the rootball is visible on the surface. If not watered frequently, they could just be sitting there due to drouth stress. If there were new roots, I'd just make sure they have good fertility and moisture and see what happens. If no roots, try splitting the rootballs a bit and replant. If you don't have room to keep them to play with and plant new ones, then toss and start over.

I have acquired the habit of making multiple vertical slits with a utility knife in all potted material that I put in the ground. Something in a 3 gallon container that was very root-bound would get 6-8 vertical cuts to the full depth of the blade. This operation helps guard against circling (girdling) roots & the possibility of water/nutrient flow being cut off later. The injury to roots also generates chemical messages that stimulate quicker root (re)generation. Though some container gardeners can come close to growing plants temporarily at nearly peak potential genetic vigor (when everything is just right), it is a good bet that o/a vigor and and a jump on getting the plant established in soil would be enhanced by planting out now. I think the main concern would be getting the plant enough water during its period of establishment. Al

Bamboo are not like other plants in that they don't benefit from having rootbound roots sliced up; in fact it can set them back. If you've got some loose roots that you can position, I guess that wouldn't hurt, but be careful not to bend them too much. You have two basic options available: 1) unpot the plant and put it in the ground without messing around with the root ball; 2) if you've got that many roots and more than one culm, you might be able to divide it in two with a root saw and have two plants. In general, don't worry about a rootbound bamboo. It will be happy to get out of the pot and will do well all by itself.

I would Repot!