Since the biggest issue with the 99s seems to be poly tension loss, maybe the optimum setup for it is natural gut mains and copoly cross? Nat gut is the BEST string for retaining tension and will help to "mask" the copoly when it starts losing tension. The key then is to find the best tension for this hybrid, since it will have a lot of power.
I am thinking around 64/58? Has anyone else tried a nat gut/copoly setup and what were your results?
Wilson Black Blade 104 - NXT Tour / Copoly at 55/51