IMO, unless you're doing snap rolls and/or gyroscopics, the move to composite should be less about engine (crank) stress and more about other factors such as lighter weight, customized pitch/diameter potential, and smoothness. A metal prop is like having a big tuning fork on the front of the airplane, and the composite prop will be much smoother and eliminate harmonic issues and possible red arcs on the tach. I moved from a FP metal Sensenich to a Catto specifically to reduce stress on my light AEIO-360 crank flange on the Pitts. There's plenty of weight in the blades of a metal prop to make a big difference. I gained no performance with the Catto, but for the way I fly, it's a no-brainer and makes certain low-speed figures easier without all that torque and gyro effect. But I'm flying IAC Advanced sequences with it, including lots of snaps.
Regarding additional stress on the engine, you can expect higher stress on the crankshaft, flange, and bearings with a metal prop when doing acro. Anytime you rotate the prop disc at high RPM (pulling for a loop, hammerhead pivot), you're applying significant gyroscopic loads to the crank/flange. If you're doing little-to-no acro, don't worry about it. And even if you are, I've never heard of anyone cracking even a hollow crank or lightened crank flange by doing light (Sportsman-style) aerobatics with a metal prop.
