Being an indoor grower myself I share the same challenge as you in keeping these trees manageable. You don't really have much choice but to prune since it's undesirable to have such a tall tree inside. Aside from scraping the ceiling and making the tree difficult to move around for maintenance, the foliage near the top would be less than optimal in photosynthesizing the little light available at that level. I think the new growth is the tree's reaction to this situation. I would prune it back to a point just above the new growth for now. Since it appears to still have a considerable height I would consider topping it at a future date to force it to branch at a lower level. However I would not do it until the tree has had a chance to recover. Hopefully Jim can offer us some tips on how to make these trees bushier over the long term. Take the tip from the pruned section and root it as a cutting thereby continuing its node count. Repeat and one day you will have a mature, fruiting grapefruit tree. I think it would be interesting and you have nothing to lose. Regarding taking the tree outside, you have to weigh the benefit against the possibility of exposure to pests and having to deal with them before returning the tree indoors.