I've actually looked into this a little with my V35. Done some 0 - 100's etc. And im pretty sure having the snow button on makes you accelerate quicker. It all makes sense you know, In the snow you want more traction - therefore, when you put snow mode on, your wheels (bigger the better but be careful not to hurt the engine) grip into the ground a lot more.
I've seen an increase of at least 8 seconds.