Here is my experience. Your mileage may vary.
Had the V&H pipes put on at the dealer (my bike was involved in an accident and instead of replacing the stock silencer, I opted for the V&H mostly because all other pipes were not available for sale in kommifornia). They also said computer was updated.... I did feel a difference in performance but very little gain in sound, as most people have reported. It did seem a little deeper with an occasional pop on decel.
A month or two later, I installed an x-pipe. I felt no difference in performance. However, sound was much better (better=louder and tone). Still have occasional pop on decel. I have not removed the airbox.