I'm not sure on the HP ratings but I know 300M is the strongest material used for an input shaft .
Looking back it was Sept 2008 when I bought the input from A-1, man time flys, and the price of parts keeps going up big time.
How fast does your dart run? Ever have a problem breaking or twisting output shafts? I have heard of others breaking the outputs...and those shafts in 300M are around $700!