I'm running the complete Pro Circuit T4 on my XR650 and it works great. This system is all stainless steel sans the canistor which is aluminum. The fit and finish were excellent and I feel it improved power delivery everwhere. :)
The Big Gun setup does improve the power delivery slightly it doesnt really add that much horsepower, it just livens up the power band a bit. The biggest advantage to an aftermarket exhaust on the XR650 is the weight reduction.
I have riden the Xr 650 for about a year now and the first aftermarket pipe I could find was the BigGun. I love it! And would recommend it. It improved both the low end and gives a little kick in the middle and top. Ive heard rumours just today however that the Pro circuit pipe is cleaner and produces more horsepower?????:silly: Either way change your factory pipe you will love the weight reduction and (Sound).