The debate, Why is a plenum more efficient than stock per-plans seal set-up? It is because soft seals, no matter how well done (RV or Cessna) will never be quite as well sealed as a dedicated top on the baffling (ie plenum). Also seals wear out over time. The second advantage is the top on the baffle (plenum) supports the normally unsupported top edge of the baffle, which my reduce baffle fatigue cracking. Just my Opinion.

Combine a plenum with a tight leak free connection the (cowl) air-inlets, you can cut down the inlet stock area, because you are using all the air, loosing little if anything to leakage. That equals less cooling drag and increased speed with no loss in cooling or even improved cooling over soft seals.

Cheers George