Hmmm...
The 2011 standard RZR now shares the HO motor that was only available on the S models in the previous years. I do know that the factory exhaust was different between the 800 and the 800HO motor, but you could add the HO header to the regular 800 if you wanted (so nothing too major could be different between the two)
Most exhaust companies offer 2 different systems, one for the S (800HO), one for the standard RZR (non HO engine)...so there must be something different between the two with how they bolt up or how the exhaust is routed. So basically, you may need the S exhaust system since your '11 RZR has the HO engine... But I could be wrong here, maybe the difference is in the routing for shock/suspension clearance instead of physical differences in the motor/mounting...I'm really not sure.
But I'm sure you aren't the first person to try to put DMCs on a 2011 standard RZR...nobody over at rzrforums stepped up to help? I put barker duals on my '09 S, and it took a little tweaking and wiggling to get that same pipe installed so it wouldn't hit the frame....but yours looks way off compared to how mine was.