If it would add and heal those 15%, it would just be a bigger heal which they just nerfed because it was overpowered. So this is a no-go of course.
Adding those 15% on top of your max hp is there to make sure you don't "waste" potential healing done by the medpack, which is quite nice after all. Every other medpack or heal will just heal you to max, and everything on top of it is lost. Not for this medpack, it will still apply the health.
Of course if you use it at lower health, you don't get that effect.