A formula I heard long before I discovered the net is losing seven pounds is roughly equal to a 1 HP gain. How true it is? No idea, but weight loss is a key to any race setup, from titanium to extra holes. Less mass means more efficient movement/less required power to move it.
Depends on where you lose the weight plus how heavy the vehicle is. 7 pounds on an 80k lbs semi tractor wouldn't mean jack but it means a lot on a 440lb sportbike.
Weight reduction on a rotating member, (wheels, transmission, engine,) is much more effective than on a static member. Also the faster the component rotates the more effective the weight reduction will be. I.E. 1 lb off the flywheel will be more effective than 1 lb taken off the wheels, which is more effective than 1lb taken off by your fancy fender eliminator kit ect.