From what pict you posted (heat sheild covers a bunch and don't see a mounting bracket), most likely is some kind of Harley Dyna muffler, either a Harley Screaming Eagle Slash cut or an aftermarket.
Do a search on EBAY and search for "dyna muffler". Then check all the pictures and compare with what you have. Pay close attention to mounting bracket as the Dyna and Sportster models mount slightly differently.
As for power, tech section has bunch of carb tuning threads.
Remember, on most bikes, first 1/4 turn of throttle is low speed mixture screw (also called air screw on some bike), 1/4 to 3/4 is the needle ( white spacer adj's that) and 3/4 to WOT is main jet.
Also, check the spark plug periodically after making a change. It is your eyes to the inside of the engine. Going too rich can foul the plug, and cause you to get stuck somewhere with an engine that is bucking and coughing (trust me on this one
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