You could also check on www.sporttwin.com. That is pretty much all buell riders over there, and a lot of very knowlegable folks on this topic.
From what I have heard, they are great pipes. It seems that the drummer differs from say a D&D, or the race kit pipe by where it puts the power. I think the drummer may make the most power down low in the rpm range rather then up high, but I could be wrong.
Oh.. and they sound b-e-a-utiful, IMHO.
Hope that helps a little.
I have a drummer on my 04 XB12R and it rocks. It makes power down low and just about as much up high as any other exhaust for a XB. To get the most power order a loud drummer. On a XB12 to get the most hp you will need a race ecm, pro series filter, and a TFI unit. Go to www.kdfab.com and ask Kevin on the forums all the questions you want. He is a great guy. The cost is $400 and a stock pipe to send in. HE will make what you want and be able to tell you what you need to do to make the most HP. The Drummer really makes a lot of deep down thump also. You might want to wear ear plugs with one on your bike.