No, they wont hurt your motor. And yea they help out some, mostly on highly modified motors, dont really do anything for stock. And I recomend the TrailBikes, its just like the Kitaco but about half the price.
I have a CF Posh unit, a Kitaco Unit, and a Trailbikes unit. I'll be doing a writeup over the next week or two when I find the time. All have been tested on a full-race Kitaco Ultrahead 108cc motor.
Let me give you the long of the short. Compare all three vs the stock box and there is a sizeable difference. The bike revs smoother, and reaches peak RPM faster.
Differences between the three? I can't tell any difference between the Kitaco Unit and the CF Posh unit. However, the Trailbikes unit feels better starting at a lower RPM and feels the same as the other two units up top.
My Recommendation: Trailbikes unit at 1/2 the price. It appears they did their homework on this unit, and you can't beat the price.