I think it is about the same as other high-end in-ears. They probably offer around 26 dB noise attenuation. Don't know the official numbers, could be great with a little graph that shows how linear the attenuation is.
I think it's said to give around 26db, but I've experienced better isolation with the SA6's than with other IEMs. That, and it's incredibly easy to get a good seal, particularly when compared with my Atrio's (which I still love, but frustrate me sometimes with its poor bi-flanges).