Re adding higher-rep metabolic sets:
Once you get to your second week of 5s, you may well find that the tight, full-feeling you had in your muscles up to that point in the cycle has started to go away. This is often due to muscle glycogen stores having less demand placed on them once you are at the heavy, low-rep end of the cycle and so they start to deplete. This is where adding in a metabolic burn set can keep the muscle glycogen demand high. I would tend to drop the load by around 20-25% and knock out a set of 10-15 reps. Aim for a bit of a muscle burn like you did during 15s.
Like Sci and Alex have said, doing a lighter metabolic set after your heavy sets isn't going to do much for hypertrophy, but, in my experience, it definitely helps with muscle glycogen storage and that isn't a bad thing.
Just bear in mind that a metabolic burn set for each exercise will take more energy. So, if you are already finding 5s tough, and/or finding it hard to take in enough cals for growth, then don't bother with them.