I dont see how a jet kit would be bad for an engine. The whole point for having a jet kit is to make an engine run at its optimum level. Someone told me that it may even give the big V better mileage. I could notice a difference right away in the throttle response. The acceleration is way better than before. I haven't raced anyone since the kit, but I would bet that I have picked up a couple of quad legnths over what I was doing before.
The stock needles <u>cannot</u> be adjusted because the base of the stock needles are flat as opposed to the kits needles which are grooved. I do not know if a jet kit would have the same results for someone who rides at a lower altitude, but for me, it was well worth the money..

 

oh it does work just as good for the lower altitude. The jet kits really seam to help with the throttle response like you said!!