Theorizing here, but a single ring piston of the same dimensions will cause a slight drop in compression since there's only one ring to contain the gasses. Two rings means twice the seal.
I guess it's not much, and the reduced friction makes up for any loss of power.
Anyhow, so long as the piston is the same height from little end to piston crown I can't see how it'll have any effect on compression.
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