I think it's important to scale back expectations for Servo. Given that Servo itself won't be ready to be used as a main engine for quite some time do to insufficient site compatibility.

Project Quantum will use only pieces of Servo coupled along with Gecko. Maybe Mozilla will fully replace all of the old single threaded code at some point in the future but I imagine that's a ways off.

Given the gradual progress of these changes it's likely that any successes in performance it brings will be copied in the other engines before they are too far ahead.