Obviously factory testing will always yield the best possible times and should be taken with a grain of salt, but suspension/tire changes in many cases, will completely transform a cars performance.
I'd actually say that it depends on the factory. There are numerous examples of Mercedes AMG under quoting both power and 0-60 times (AMG says 4.2, it's tested by R&T or C&D as 3.8-4.0). I do completely agree that small changes - even just tires - can absolutely transform a car.