I believe there is a bit more than a couple of percent gain going barebulb. That;s really what this is all about. Trying to get the absolute maximum return for each watt burned. Vertical is much more effecient. Less heat = more lumens. The additional heat generated by a reflector is energy that should be going to your plant.
RWs point is valid, You are certainly not going to get the same lumens off of each 1/4 of the bare bulb as you would the same footprint with a reflector. Understood. But using 100% off the available lumens, generating less heat in the process can only result in an advantage.
Of course, skill level plays more into yeild than where your bulb is hanging. If you look at Heath Robinsons various styles of grows, and there are many, you will see he always has an above average yeild. To me, this indicates that good timing, plant and nute management are more important than style.
check out my vertical grow:
https://www.rollitup.org/grow-journals/263979-sealed-water-cooled-blueberry-coco.html