Just some stuff I found while researching. If anyone would like to add their experience this could be an interesting thread.
From "Fix My Hog":
Dual exhaust Vs. a 2 into 1

Tags: Exhaust It really depends on how you define power. A lot of information available boasts peak power numbers, these numbers are usually at an RPM range that most bikes rarely see. To really understand how a system performs you need to look at a Dyno graph all the way through the RPM range. There are a lot of factors that affect where a pipe makes power. Engine size, compression, cam timing, cam overlap, valve size, and intake size are all part of the equation. The biggest factor to consider is at what RPM range the bike will be ridden at consistently. The fact is that pipes designed for race bikes, which are ridden in a very narrow high RPM band, will not perform on a street bike. A general statement that can be made is; longer smaller diameter pipes make low-end power and shorter large diameter pipes make top end power. One other design constraint to remember is styling. Everybody has his or her own opinion on what’s going to look good on his or her motorcycle. Drag pipes just wouldn’t be drag pipes if there was a collector stuck on the end of them. With all of this in mind, there are a couple of fundamental differences between 2:2 and 2:1 exhausts when it comes to design. The advantage of a 2:2 is that the pipes are separate so the length can be fine tuned for a specific power band. You can also tune each pipe length per cylinder if necessary. Also, you have far more styling latitude. The disadvantage is that the power band is narrower. The advantage of a 2:1 is that you tie the cylinders together giving you the ability to take advantage of the natural scavenging affect that occurs between the cylinders during the intake and exhaust valve cycles. This tends to give you a far broader power band and greater overall performance.

AND:

 

Damn, I'd sure like to get a set of these for my FXR. They'll be on my wish list. No hurry, hopefully I find a sale on them somewhere at sometime.

 

Do you have to pull the whole exhaust to get to the rear axel or is there a removable tip?

 

No argument here.
My understanding has always been that a collector type system will provide a broader, flatter torque curve, and "might" give up a few hp at the upper end of the rpm range when compared to a dual system "of the appropriate length and diameter".
Basically, duals are for the track and single is for the street, and even that could be argued, depending on the R&D put into a specific design.
Nothing that already wasn't said, was just telling it how it's always been told to me
All that being said, I think sometimes it comes down to splitting hairs over a few lbs. of torque/few extra ponies. A well designed dual system w/ an integral crossover (vance&hines big shots) have put up numbers close to popular 2-1 systems. My .02

 

I flipped my axle so that I don't have to mess with the exhaust regardless of the design.

 

You might be waiting awhile. My understanding is RBR pretty much builds to order. Might get lucky and find a used set, but I think most keep 'em when they get 'em.
They are some really sweet pipes, though. Just a little too rich for my blood...

 

If you guys get to feeling sorry for me you could pass the hat so I can get these. I would be very grateful.

 

A simple example of the benefits of a 2-into-1 are the Buell models. By using equal length siamesed pipes Buell was able to raise the level of tune the stock engines are built to, with higher CR and more aggressive cam profiles, producing a wide torque range and much bigger numbers than a Sporty. In addition the bikes also meet the same EPA regulations.

My own experience, with my S&S 107" engine, involves a set of true duals (Samson pipes with S&S oval mufflers), which I later changed to a Supertrapp Supermeg 2-into-1. The reason was simply that I got fed up with the left pipe of my TDs resting on the top of the primary! I had my bike set up on the dyno for both, by the same shop and operator.

The headline figures for both are similar: TD 103HP, 102TQ; 2-into-1 101HP, 107TQ. However what those numbers don't show are the gains in low/mid range torque of up to 15%, between 2,500-4,000 rpm, where most of us ride. In addition the engine is far smoother running with the 2-into-1 and a much nicer thing to ride. So there are measurable benefits, plus tactile ones!

Clean and dirty:

 

Yep, always run an exhaust with a collector, car or bike!

I don't have any official numbers but my seat of the pants dyno is giving the Samson powerflow 2-1 good marks, especially mid to upper end!

 

That is good to know as I just put a set on my bike!!!
Haven't had time to run it yet though..