As long as you don't have any aching joints or similar discomforts, skip the 15's. You'll save time and keep yourself more motivated to workout simply because you get to do what you want to do. This won't affect your growth in the sense that your muscles won't be less affected by the rest of the workout simply because of the 15's that you skipped.
Like Totentanz recommended, if you do skip the 15's, you can extend the cycle if you still want to keep an 8-week cycle. If you really want to squeeze the most out a cycle, extend the 5's (using your absolute 5RM) for two weeks, then go to the post 5's and you can extend that for as long as you can keep the progression steady and gradual, plus two more weeks for the absolute max you want to train with. To make sure the muscles get enough metabollic work done for the really heavy weights you have to work with at the end of the cycle, you may choose to do metabollic sets (I recommend them over dropsets simply because they are simpler to do, especially without a partner to assist you).
If you even want to squeeze even the last drops of growth during the heavy weights (post 5's), after you determine the load progression, repeat the weights for two or three workouts before going on to the next progression in load.
As for the negs, as long as you can do them properly, you don't have to worry about doing too much, so you can practically go crazy (within reasonable limits, of course; for example, don't take what I said and literally go crazy and do 75 reps for one negatives exercise), since negs aren't taxing to the CNS (i.e., much little danger getting CNS fatigue). You can do your negs after the post 5's, and similarly repeat a weight for the negs for two or three weeks before going on to the next progression of load.
You can end up having a cycle that is 10-12 weeks long even after skipping the 15's by following the advice above. You make sure the load you work with is still effective, thus producing still some hypertrophy. How significant hypertrophy that is, well, we can all argue about it, but the best way would be to try it out for yourself if you want to. The strength gains, however, especially during the negs, would be significant. I remember a post from Dan so long ago, saying how he kept extending his negs simply because he kept on getting stronger.
But take into consideration that, after such a long cycle (almost twice as long as the basic recommended HST routine presented to newbies), your muscles will be very much conditioned that it may take you a longer SD in order to properly unadapt your muscles.
Hope that helps. Good luck
-JV
