Don't remember the exact source, but one study put 3/4" lower head/torso position at saving 10 watts.

This was at a given, moderate speed.

A very aggressive position will compromise power output for most folks, but there's likely to be a range before getting to that point.

If you want to go faster, there is a natural tendency to get lower.

It depends on the goal- if you want to go fast or far, then efficiency is important. If JRA, sitting up is better to watch the scenery.