I used to view 15 reps as worthless. I spent about 6 month doing SST and got a little burned out there. Coming back to HST I really enjoyed 15s, they are like a mix of cardio and weightlifting.
Anyways, one thing to keep in mind when you're planning your rep ranges is that they are just an easily communicated vehicle for describing the target intensity ranges. Your intensity range should in general be trending up throughout the cycle (even if you zig zag). Because we cannot just do endless work you can sacrifice volume as intensity goes up. So doing a 12, 10, 8 (just an example) is frankly all going to be similar intensity ranges (70-80% of your 1rm) and fairly similar volume ranges.
However HST it trying to take you through a range of intensities. After an SD your muscles should be sensitive to a load so a small intensity should suffice for a growth stimulus (i.e the 15s) as you progress you will need to continue to increase the intensity to trigger a growth stimulus which is why you work your way up in weight. So if you want to skip your 15s you should probably do more than just extend the 5s and maybe go into doubles/triples and/or negatives to create a continually ramping up intensity throughout the cycle.
Personally, for this cycle I chose to go 2x15, 3x10, 4x5, 6x2. What this works out to is that at the begining of the 15s I am working with 47% of my 1rep training max (42% 1rm) by the end of the doubles I will work with 98% of my 1rtm (89% of 1rm). Not that I am recommending this, just using it as an example of how to work through the rep ranges. Anyways I hope this helps.
