>To say microdisplacement doesn't work at all with the superfly engine isn't exactly true. It depends on your root node. It works with the poser root node and the physical surface root node.

Are you sure about that? And it's not just that your spheres have more polys than you think? Because I have used displacement with PhysicalSurface and I needed to raise the subdivision level a ton to get it to work on some props. I would have preferred not to because it cost a lot of render time.

>>An up-close render of the 4th ball shows there is actual displacement going on, but I see no way of controlling the strength.

>Even with a multiplicator or something like that?

Yes, all you have to do is take your displacement source (e.g. an image map) and plug it into a Math node, and then plug that into the Displacement input of the Cycles root node. I generally use Multiply. You can then raise or lower the amount of displacement by multiplying by more or less.