As long as you have closed off ventilation, meaning the case is completly closed, you should notice a drop.
If there are any at the front, place these to blow air in (feeling them, the ones blowing in seem like they aren't blowing as much) and place the ones at the back to pull out. If you have ones on the side, also ask these to pull in air to help the motherboard cool down.
The hot air from this is then pumped from the back since thats the only way to go, helping circulation.
The ones at the back should also be the biggest, the ones at the side i'd say the smallest to not mess with airflow as much, and the front to be one of the biggest too if you have any
Hope this helps. Just think of the way air passes through, that should help.