Often the car magazines (C&D, R&T, Motortrend, Automobile) will compare the quarter mile times of their long term cars from when they first received the car to when the car has ~30k miles on them. Just about every time that I recall the numbers with the car being broken in are about ~.2 sec faster. They usually attribute the gain to the engine breaking in and less friction in the driveline.
This is far from scientific so take it for what its worth.