Don’t know what the hp level they are needed as there are several factors there.
The shaft obviously holds over 1000hp, that is if everything in the transmission is working correctly. I of course run this shaft, since installing it in my transmission it has been out three times. The last two of which was for P2 replacement. The factory shaft relief was left in the design this was done to allow a break below the spline in case of the transmission locking up with a binded shift or failed planets ect… The idea was to prevent as much transmission carnage as possible. I have found Allison parts are expensive and hard to get when I need them. The 550 price tag of the billet input is well worth the piece of mind.