I really don't see why everyone is saying it'll be an issue. There are plenty of alu shafts kicking about.

Your increase in acceleration is most likely as you'll be loosing less power to the driveline with it being significantly lighter, thus having lower inertia.

Shafts transmit power, and hence their rating is in power IIRC. This will most likely be stated at a speed and hence you can work out what torque is.

JAmes.