Hi eleven11,
It is important to remember that in the case of the 15 rep block, it will only serve to promote hypertrophy if done following Strategic Deconditioning (SD). If you abruptly go from training to failure with weights in the 4-8 rep range to using 15 reps with only 80% of your 15RM, you aren't going to "feel" much of anything but warmed up.
The 15's will only create a noticable hypertrophic effect after SD. Aside from that, they will help to improve the metabolic capacity of the tissue which will indirectly help you grow. Altering the metabolic environment inside the cell is anabolic so some extent, however, it won't lead to satellite cell activity, thus it will only serve to increase contractile proteins within the nuclear to cytoplasmic ratio limits. In other words, you may build a little muscle, but the volume of the muscle cells won't change noticably.
Anyway, without going into too much needless detail, the 15s will not make you grow if you are already highly conditioned.