Chatter is usually caused by a warped flywheel, at least initially, and off-the-car you can detect the warp by the blue pin-spots (up to ~1/2"-3/4" in diameter on the surface. You can also straight-edge the FW and check it that way. If you allow the chatter to go on too long, you eventually do damage to the entire drivetrain, especially if it's severe chatter.
At stage 2.5 I have not been happy with my WRX/ACT setup and am going back to the stock setup which worked very well for 100K miles. WRX
FW and ACT HD clutch never felt right from day one and I'm really tired of the associated noise and engagement chatter on launch. It grips well when engaged but it has really taken the joy out of driving the car when all the other things are considered. And I have the TSK3 on as well, which I will likely leave on when I change out the ACT setup.