Depends on the engine and installation, but a good plenum design can even out the cooling airflow across all the cylinders as well as improving cooling for all of them a little bit. It can also make for huge decreases in drag because you can use smaller inlets to direct the air where it's needed.
The designs we're used to looking at on the legacy planes (Cessnas, Beeches, Mooneys, etc.) were really lousy cooling designs. They didn't know any better, so they made huge gaping holes out front and hardly left any place for all that high pressure air to go out. A lot of it just spills out of the front of the cowling. Take a look at LoPresti-designed cowlings, like on the Archer III. They're little tiny holes barely as big as your fist, but they allow the right amount of air in and give it a good escape route.