I'm not sure the percentage, but I know when my abs are REALLY visible, like deep depressions between the muscles, when I sweat, it smells like ammonia, so apparently my body is breaking down my muscles to use the protein for energy at that point.
So, just anecdotally, I'd say, yes, you perform better with some body fat based on my experience. Where that percentage lies.... I'd say probably closer to 10% than 18%. When I did GOMAD, I was probably approaching 20% and my performance was not so hot. I seem to PR most often when I'm at that point where the two top abdominals are just barely starting to show. I get leaner than that and I start to bonk on longer workouts and runs.