don't BC's come with 7kg and 4kg springs standard? From memory that's around the same rebound rate as stock. Performance gain? Turn the dampening off full firm and see how the bounce rate is. The midrange of most cheap coils are terrible, they only work properly at either extreme due to the nature of their valving. Add to this that BC's don't have progressive springs, and it's pretty much a recipe for fail.
With the same amount of money you could get a very good set of matched springs and struts that will last longer, perform consistently and much more smoothly. Fuck adjustable dampening and height, the only guys who do that usually have $5000 suspensions and visit a suspension shop before and after each event. If someone's on a $1200 budget for suspension their broke ass isn't gonna see a suspension shop after it's been set. ever. they'll just ghetto it up at home every time they get defected.