If done properly there is nothing wrong with building an 8ft wall, or precut ht, and then building a a short wall on top., The benefits are no 12ft studs to handle, and they are more likely to bow or twist, deal with some nice straight pre cuts, then build a nice gable wall on top, no strenth issues, this is an interior non load bearing wall. G, PS even if it was load bearing, NO PROBLEM, ALSO, forget the space up top , bring top plate right up to cieling. The A job is to cut the existing rock away, block out as required, attach top plate, you can also layover the existing sheetrock, but if you are between bays , not such a good idea. G