I think performance of the style systems in Blink and Firefox are similar. Since Servo's style system is linearly scalable, we expect most users to get a ~4x speed improvement on styling.
Users's don't care specifically about style performance, though, but about things like interactivity. We think Servo's style system will improve those things, but don't have numbers for that on hand.
To give a concrete example, it takes Firefox ~1.2 seconds to restyle the single page HTML5 spec with it's current non-parallel style system. Firefox with Stylo can do this in ~300ms. That is close to a full second off of initial page load.