The drag of an O-Ring chain will lower the mileage.
I changed the chain on my Enertia from standard to o-ring and did not notice a change in my range. I say that with some confidence as my commute follows the same route every day and I arrive home with a state of charge that varies from 28 - 40 percent. That may seem like a wide margin, but it's always been that same margin. I attribute it to wind, temperature, and riding style.
When I changed to an o-ring chain the noise reduction was amazing. Because of that, I was predicting that my range would actually increase, probably because I was attributing the standard chain noise to increased friction, to a resulting limitation in the range. I was wrong, however. The range has not varied at all. I will say, however, that the lack of sound from the o-ring chain will probably make you FEEL like you are sailing along.
If Harlan is correct that you are not able to use an o-ring chain (and I have no reason to doubt this very knowledgeable dude) then, that is unfortunate.