A 6mm arm blocks 2.4% of the prop disk, followed by 6.3% and 9.3% for 15mm and 22mm. There's no thrust data for an arm-less configuration, so I compared them to each other. When you compare 6mm to 15mm, the 15mm has 98% of the thrust, and 96% prop disk area. 6 v 22, the 22 has 95% of the thrust, 92.9% of the area. 15 v 22, exactly 96.6% of both. So it seems the two track pretty closely (duh). An arm that's considered 'pretty narrow' could still cost you 100 grams of thrust when you add up the four motors. Flaco vs Mixuko? About 280 g. This shows how it's silly to obsess over 2 or 3 grams on a frame when there's hundreds of grams of thrust to be gained.

Fairings

The full Flaco fairing, including a trailing edge cone for the motor, yielded 1416 g vs 1408 g for the bare arm. Unfortunately, the fairing weighs 10 g. This isn't the best test of the concept of a fairing, however, since it has almost double the frontal area of the bare arm (it has to wrap around the arm, after all). The glass half-full interpretation of this would be: the Flaco fairing must be a really good shape, 'cause it increases thrust by 8 g even though its frontal area is much bigger. If only it could be made much lighter.