I did a little testing and found the following results, using r208, Chrome, i7 with HD4600. The test was a fps throttle, target between 51-56 FPS by creating/destroying horizontally moving sprites (sine behavior).
Sprite image with zero opacity (animation frame 1):
-- B to F: 2260 sprites at 51 FPS, 9% cpu
-- F to B: 2900 sprites at 56 FPS, 9% cpu
Sprite image with approx 15% of surface < opaque (animation frame 0):
-- B to F: 2290 sprites at 51 FPS, 9% cpu (partial opacity makes no difference - as expected)
-- F to B: 1630 sprites at 51 FPS, 11% cpu (significantly lower performance)
Here's a link to the capx.