Note that the 'wet noodle' frames didn't seem to slow Sean Kelly down. And the structural problems with some of the frames were due to the fastening method at the joints - not the frame material.

But I think you're missing the main point. If one can build Al frames noted for their comfortable ride relative to steel, albeit at the expense of some additional flexing when pedaling hard and one can also build Al frames noted for their stiffness when sprinting relative to steel, albeit with some sacrifice in comfort then it's not the choice of Al vs. steel that determines comfort vs. stiffness but rather the design choices made in the frame geometry and the tubing sizing.