Save your money. It keeps the power band exactly the same as it is, but 200 RPMs lower. I tried it with the -30 FMF rev pipe hoping to reduce the hit. Didn't work. My -35 torque pipe would actually achieve a higher speed than the -30 with the torque ring. The biggest difference it made was to the sound. It sounded more like a 250 with the ring.
Well I have a 2000 KDX200 with the knarly pipe system on it and put the 5mm(I think). I havent really been out in the woods as of yet but from what I have ridden it, there was some change. It seemed alot more responsive on the bottom end, esp in cornering. I think for the 8$ I spent on it, I will be more than happy with the results.
Check out mxbonz.com , there you get 2 rings , a 3mm and a 2mm so you can "tune it" how you want. I have a CR 250 and put the 3mm in and it is working well. Maybe later on down the line I might stick the 2mm in with it to see how that does , making it 5mm altogether. Good luck