From what I have been able to figure out by looking at pictures of various diffusers, the Noblesse unit is the only one that comes anywhere near being a 'real' diffuser. It appears to be made of two parts; a box-like unit that smooths the center of the car from behind the rear axle back to the bumper, then a second trim piece that fills the gap between the boxy piece and the bumper and mates up to the bumper. Only the the trim piece has diffusing fins so I'm not sure how effective it would really be.
Oops never mind. I forgot I was on fitfreak, where looks aren't the main thing, they're the only thing.
