Does seat post suspension reduce efficiency?
Seat post suspension is not designed for efficiency.

It is designed to add comfort to a hard tail mountain bike frame. There are only a few really good suspension seat posts, and they are rarely used on hybrid bikes. Almost always, you will find that the suspension post on a basic hybrid is just a spring in a tube. 

Occasionally, they add the ability to pre-load the spring to make it feel stiffer, and that is a slightly better option. 

On that type of suspension post, you affect your efficiency when the post is active because your seat height will change as you ride, and as the post compresses. This will change your position and leverage, and usually, not for the better. It will detract from the amount of power you are getting out of the system, while still using the same or more energy to put in that power.

A true full suspension will account for most of these factors, and is therefore generally a more efficient choice, but weight, cost, and designed riding style for a bike will also affect the efficiency.

To directly answer your question, yes, you would be more efficient without a suspension seat post, assuming all else was equal. 

However, most hybrids are lacking in any number of areas in regards to efficiency, and you may not feel the actual difference in efficiency from the seat post given the bike you are riding.