Myth: Will low light reduce spawn rates?
Solution 1:
Now I built your example in 1.8 (I'm guessing You're using 1.8) And my results were... interesting.
Now first the mobs did spawn inside this little room as you can see by this image:
Now if you consider that hostile mobs only spawn if the light level is 7 or lower then yes. In theory this is true. BUT, In most cases of me just removing the glowstone and just me standing looking at how many spawn I would say that having no light source helps but it doesn't help a whole lot. I hope you found this helpful for whatever you're doing with this =D