I did the DIY AB mod before buying the S&B. You can tell a very very slight difference in power and noise with just the DIY mod. The S&B is a big difference. Its pretty quiet in normal driving, but when you get on it, it can deafen you (in a good way). HP wise, i dont have any specific numbers, but its def more than the the DIY mod. Well worth the price IMO. they're pretty cheap as well. An as Southside said, all three make a big difference.