Just wanted to throw my .02 in since I recently purchased and installed one of these coils. Since I put this coil on, my banshee starts easier, runs smoother, and does accelerate quicker. I can't go as far as saying you'll gain "x" amount of HP with one, but I have noticed a positive change in how my bike performs. I won't know if or how much HP gain I received from it, simply because I don't have any dyno time before the coil....with the results that I have first handedly experienced, I cannot imagine losing any HP with this coil. You may not gain much or anything, but I simply cannot fathom taking a loss.
I'm by far no expert, but I can vouch from first hand experience that this coil did improve the performance of my bike.