That would be a 1.3% drop in weight on a 38# bow, and a 1% drop in weight on a 50# bow. If by “efficiency” you mean arrow velocity, we also have to take into account that changes in bow weight generally have less effect on arrow velocity as bow weight increases. Therefore, I would imagine efficiency would be affected more in the 38# bow than the 50# bow.
As to whether you’re splitting hairs, that would depend on whether you could tell any difference when you shoot them. I don’t think I could tell any difference in a 1/2 pound drop in bow weight, but somebody might be able to.