To do a full rotation of pws in 10man you need (1.132*9 + 1.5), 11.688s for GCD's and a flawless connection 0ms.
PWS will be up for 15sec, This gives you 3.312s of "free" time befor you need to start the rotation again. If you want to pre shield an ability you no longer have the power to do so, unless the tank doesn't take any damage.
While we at it, the RE [Strengthened Soul] reduces the Debuff of PWS by 1s stacking. If you use this you will have 0.312s of free time, add latency and you now have an RE that wont have its full potential at 3x.
You have made PWS a 1 button spec.