Jules,
When the loads are light (lower intensity) at the start of the cycle, it's very easy to do a ton of work (ie. a lot of N/m!) without overly taxing your CNS, esp. if you stay away from failure most of the time. Doing 3 sets for 15s doesn't seem like a big deal (unless you have a shed-load of exercises to do - not recommended). It's a lot of work but at a relatively low intensity.
However, unless you get close to failure with the light loads, you aren't going to be maximising motor unit recruitment. Maybe you would need to get 10 reps with your 15RM load before all the available motor units are activated (due to fatigue). So, for a 15RM set, you might only get 5 or so reps with full fibre recruitment. For lighter loads, where you stop short of failure at ~15 reps, you might not get any reps with full fibre recruitment.
Does this really matter during 15s? I don't think it does. 15s have several benefits besides eliciting a PS response. Of course, if you are deconditioned at the start of a cycle (as recommended for an HST cycle) then you might well get a PS response even with light loads – something you probably wouldn't get if you just dropped straight to 15s from heavy loads.
As the loads increase over the course of a cycle, and therefore the intensity of each rep increases, your CNS will start to take more of a beating. If you then try to do too much volume (ie. total work), accumulated fatigue will quickly build and you will find your strength dropping off.
So, it's a question of balance. You have to figure out what you can manage. Once you get to loads that are close to your 10RM, 20 total reps would probably be a good place to be aiming. Then, during 5s you might find that 4 x 5 is too much (particularly during the second week of 5s) so you might drop the sets down to 3. If you can handle 4 sets and recover well, then go for it! You'll probably get a slightly better PS response. (Bear in mind that if you do more than one exercise per bodypart you might well need to approach this differently.)
With varying volume over a cycle, you could try something like this:
3 x 15
2 x 15
3 x 10
2 x 10
4 x 5
3 x 5
For post-5s work towards new 5RMs.
(For RM workouts I would drop the reps down for subsequent sets if need be, eg. with a 5RM load, I might do 5, 4, 3, 3 to get all 15 reps).
Doing something like this worked pretty well for me. Along with a bit of zig-zag between mesocycles, I never felt too fatigued. I would have four main compounds per session and a few isos.
If you are concerned about work dropping off at the end of the cycle, you can always add in a back-off set here and there. Worth trying without though.
