I find it amusing that there are so many different opinions on the dual exhaust on the 800. From a logical standpoint John Q Public looks at the exhaust system and says " Hmmm...2 pipe into 1 pipe then back into two silencers. Must be a fake". Or "Polaris must have done this to limit horsepower due to a government limit,etc,etc...."
Some reasons why this system is set up this way.
1) The single "crosspipe" flows the same CFM(Cubic Feet per Minute) as two individual pipes, so mathmatically there is no loss of volume capacity.
2) Vehicle design often constricts engineers and vice versa. The 800 uses the same platform as the 400, 500 and 700. Therefore routing a seperate pipe down the left side of the vehicle was un acceptable and why have 2 individual pipes running sideby side when you can have 1 larger diameter pipe that flows the same amount of gas(air).

Folks wringing their hands about the so called "fake dual exhaust" are not looking closely at the system. Also, the silencers on a fourstoke are as much an influence on a system as the pipe layout and design. For proof look no further than your local motorcycle shop. There you will find 4 Cyclinder Sportbikes whose exhausts are (gasp!) 4 into 1 ! Why? These motors are high RPM in line cyclinder designs which make big horsepower in the high rpm ranges but don't make alot of torque down low. Want proof? Pick a bike, any new 600, 750 or 1000cc machine. Take it out on the road and whack throttle open at 4-6000rpm. Sure your get some power (more with the 1000's VS the 600s), but not alot happens. Now try the same experiment at 8-10,000 rpm. Now go home and clean your newly soiled underwear. The reason these bikes have 4into one systems, is weight and clearence issues, but more importantly they help these engine designs make torque at low rpms.
But wait!, you say. I saw twin cyclinder bikes on the same showroom floor that had dual exhausts! So there!
Not so fast Charley. Twin cyclinder designs as a general rule make strong low rpm torque but fizzle out at higher rpms. So therefore having 2 seperate pipes allows for more horsepower to be made as the rpm increases. Also the manufactures have found that customers like the look of seperate exhausts. Form does not follow function.
Look closely at some of the big V-twin crusier "Harley" type bikes. Behind the chrome exhaust covers you will often find a crosspipe hidden. This balances the exhaust flow between the 2 pipes and makes up for the fact that one pipe is shorter than the other. Again, not a true dual exhaust system.
So, back to the 800.
If you put high flow silencers on you will most likely gain top end horsepower, at the expense of some bottom end torque and with additional noise. To compensate you will then have to increase the diameter and change the curve of the header pipes. The length and bend of the header pipes will also change the horspower and torque curves. Some companies are even experimenting with "bulges" in the header pipes about 6 inches from the cyclinder head. Look at the Powerbomb header pipes from FMF (may still be in prototype stage,i'm not sure)
All this being said I am curious to see the " true" dual exhausts the after market will be bringin out. There is more power to be had in the 800, but the exhaust is not the biggest problem.
Clutching,airbox mods and remapping the ECU(IE. using a Dynojet type box) will also be important.