I have ****s racing counter bore and the intellijet on my 250SX. It is fantastic. Not sure how it changes the power curve but it feels like it hits a second gear within the gear. When it comes on if I am hard on the throttle it will pull the wheel off the ground in 3rd and 4th. You also can change the jetting with a simple dial based on temp or elevation. And yes it is pretty much right at 500, was worth it to me.
The advantage of what they do is they leave the opening 36 and conter bore it to 39 so you have less jetting, idle issues with the added power of the larger bore.
Never heard it becomes a leaky mess, mine is not, but I am pretty meticulous with maintenance.