Umm as far as a noticeable difference over stock... My buddies started wearing earplugs after I got the HMFs! They are LOUD. Too loud for some, but I love them. I used to do my best to leave my neighborhood at under 2500 rpms so as to not disturb them.
Best sounding pipe I have heard on the busa, but MSF and others say the HMF big bird sounds better, so I take their word for it. D&D are just as loud as the HMFs, but its very raspy and annoying, not throaty like the HMFs.
FYI The dual HMFs are louder than the Hindle at idle and at speed. Yes I have heard both.
Both pipes have stepped headers, which make really good power.
They get you to virtually the same HP, around 168-171ish depending on your bike if you have it tuned properly.
The dual HMFs cost a touch more than the hindle, but you are getting more pipe. On the other hand, the Hindle being a single pipe weighs less.
If sound is an issue, and you like loud, I PROMISE you wont be dissapointed.
I wish I weren't on 56k, I got a good MPEG of me coming out of a corner at about 7K RPMs with em.