I think there are two different problems. One is strength of rod bolts. On over rev the rod bolts can be stretched which causes spun bearing.
The other problem with spun bearings on 3.2 seems to have more to do with loss of oil pressure at high RPM. Normally the oil is only supplied to the front and rear of the crank. This is one of the reasons to have the center main bearing drilled. This modification adds an additional supply point in the middle of the crankshaft. Normally the oil has to overcome centrifugal forces to get out to the center rods so pressure goes down as RPM's go up.
The picture below if from a old Upfixing that shows oil supply for 911,908 and 917. 917 had oil supplied to center of crankshaft. You can see in diagram at top (911) that the inner rod bearings do not get as much oil at high RPM with the stock oiling.
This seems to be less of a problem on the 964 and later engines. But these also have a much larger oil pump than the 3.2. Apparently, the oiling path and size was changed for the GT3 cranks so that the center oiling is not required.
john