At a rough guess, and never mind better heat dissipation due to things not being "enclosed", there could be a 5% increase in available "light" as the plastic, even when clear, will absorb some of the light and therefore attenuate the potential output. Glass does the same, no matter how good the quality.
So unless there's an issue with IP rating, which I doubt as you won't be spraying water directly on the light, then you're going to be better without the plastic cover