To be black diamond at IS you need 200.000 downloads. To get maximun royalties --let's say "plain" diamond--, 25.000 is enough.

Probably if you upload everywhere, five, ten different sites, you will earn more. But I turned diamond at silver and never looked back. I doubt I would earn much more spreading my portfolio through the whole web.
Maybe more, it's possible. Much more, I don't think so.

As for SS, yes, it's true that they have a very small profit margin, even less if you count referrals, but I suppose that if they have worked that way for three years, they will be able to go on.