Pro circuit pipe spark arrestor. I didn't like the spark arrestor that the pro circuit pipe came with as it was like a quite core insert and they don't make a performance model. So I ended up getting a FMF 4.1 spark arrestor and carefully cut off the rings that hold the screen on without ruining the screen. Then found a perfect piece of 1 1/2" muffler tubing at Autozone on the shelf that got pounded in the end of the exhaust. I left a little bit of an overhang in case it needs to come out with vice grips.
Mine does not have the 'smaller' insert/spark arrestor.
Have you ridden with the steel clutch basket yet? Results ?