It is very tough to make generalizations about what was required for specific engines to be up-rated for higher boost pressures. For instance the Merlin needed a stronger supercharger drive shaft to make 18lbs of boost. The older supercharger drive shafts (or quill shafts) weren't strong enough to handle the power required to drive the supercharger to get the higher pressure even if the crankshaft, crankcase and propeller shaft could stand up to the power. Sort of the same for the Allison. The drive gears for the supercharger wouldn't stand up to the load needed to run the 9.60 gear ratio without making the gears wider (thicker). Of course the Allison also used 4 different crankshafts and at at least two different crankcases over the years so it is a little difficult to pinpoint exactly which modification allowed for exactly how much of the allowable boost in WEP at any given time.