Since the spine of the arrow is stiffer than that of the tube, the tube itself isn't going to add any significant stiffness, because the weight is distributed over the length of the shaft.
You might find it necessary to use a weaker shaft due to loss of performance from adding the weight. Note I said "might".
Some folks have reported they were not using the tubes full length. They are cutting them down to give the exact weight they want to add, and that most definitely will effect spine, because of the unbalanced addition. If they are adding it forward, it weakens the shaft. If they are adding it at the rear it stiffens the shaft, and unless you epoxy them in place (and even then) there is a potential risk of shift.
Just as a side note, and a suggestion for another option of adding weight - you might look into using the Goldtip weight system. It fits the CE shafts, and gives you the ability to accurately adjust both weight (front, and aft), and spine without the worry of a possible shift, or the potential of noise created from the tube recoiling inside the arrow.
I use the GT weight system exclusively, because I have tried all the others, and for me the others come no where near measuring up.
