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The post rewards are calculated in USD terms. That has nothing to do with anything unless you're cashing out.

The post rewards that show on the frontends is trying to be USD (fix the peg lol), but the post rewards are just determined by how much of the rewards pool you get, and then calculated to USD.
That rewards pool is in steem. So you don't get more steem for the exact same post in the exact same environment if only the price changes

The portion of the pool allocated to you is the same. But more that is in STEEM, as opposed to SBD.

Yes, you could always sell the SBD for STEEM and power it up, but you're never guaranteed to get the full value from the markets.