i think the 15 rep total is more of a coincidence between 5/4/3/2/1's and hard 5's. '15' isn't a magic number and each HDL stimulates in different ways. extended sets work great as they are. i actually had better progress when i went heavier (5-6 reps then 2-3), although i didn't try it for that long! -i prefer hard 5's over all else, i think you can focus on it better as you know the reps you're going for.
the take home point is just to make sure you're working hard and getting all the reps you can (with decent loading). CT's layer system has actually taught us that lower rep schemes could be the future for hypertrophy. so don't worry about getting 9 reps at the start of each extended set, just focus on getting solid mechanical work in, and finish each set knowing you pushed hard and your reps were solid.