Maybe I am writing that only from my personal experience but in my opinion Sustained Power Build is not a great plan (it was not in my case). It is very taxing and gains were marginal. I tried only threshold and SS workouts and the gains were more substantial. I put little post about threshold riding here.
I have improved my aerobic side, endurance and FTP a lot. Try longer threshold 2x a week or SS 3x week +Z2 rides. But as I said - this is only my experience, but I believe that doing 3x20 at threshold or even SS gives you more than 5x8 min supra-threshold an it leaves you feeling a lot better. What is also nice - aerobic conditiong will help you massively with those efforts around 5 minutes and even VO2 max so for you it is win-win 