Will I lose horsepower by running a sealed ring chain?

The answer is yes and no. It is true that initially non-sealed chains do produce less friction when compared to sealed chains.

However as time/rpm increase, sealed-ring chains significantly outperform non-sealed chains.
During normal use chains heat up and expand; this expansion can create friction resulting in horsepower loss. Due to the increased internal lubrication of sealed-ring chains, friction is reduced, allowing the chain to more freely transfer horsepower to the ground.