Back pressure is created by anything that restricts exhaust flow. This includes: narrowing of exhaust pathways, Bends in the pathway, & restricting devices like: Catalytic Converter & Mufflers.
Just looking at the Slipon alone, it would seem to reason that a slipon with a mild diameter in the midpipe and muffler would produce more back pressure than necessary. You're right that with the stock headers (and most of the time a cat) the difference is probably negligible.
But like I said previously,
The difference is when you combine any slipon with headers and other mods. A poorly designed slipon can eliminate any performance increase you might have gotten all together. Where as one designed for performance will provide some pretty impressive results.
Just looking at the diameter of the mid pipe, the bends and the overall length of the muffler, there's quite a bit that a slipon can do (when implemented with other mods).
The more back pressure at the exhaust port, the more energy your engine will use to expel those gasses, power lost and more resistance (wear) on your engine.
A stock system has about 2-4psi at the exhaust port. The midpipe off a two brothers exhaust for an 07-08 600rr has an average diameter just smaller than that of the stock exhaust. This will surely create more back pressure, if not, it definitely doesn't reduce it. The Akrapovic has a midpipe that is nearly .5" bigger (not at the connection point between the headers and the midpipe, but overall).
Without even looking at the length and diameter of the muffler, this already tells me that not all slipons are created equally.
Your best bet, performance wise, would be to get a full system. If sound is all you're going for than it really doesn't matter. If you ever intend to get headers, which would only serve the purpose of increasing performance, I would suggest keeping that mentality and getting a slipon that would serve this purpose.