I think I heard somewhere that shaving 70lbs is worth like a tenth. But any reduction in weight WILL make you faster. If you're running at the track with a full tank of gas and the spare and jack still onboard versus 1/4 tank and no spare, yes, you'll notice the difference. That's somewhere in the neighborhood of a 120lb difference. Just my 2 cents.
70-80 pounds is roughly a tenth and 1 mph, I think the spare and jack come out to about 40 pounds all together, so that and your rear seat will shave a tenth or more.
i noticed a difference.. your rear tires loose traction and it handles differently, in a good way i'd say, but then i filled it up with a little weigth
The rule of thumb I remember is -100lbs or +10hp = -1 tenth. It's all guesstimates.
Any weight reduction is a good thing, but it would be a bigger help to get that amount of weight off of the nose of the car rather than behind the rear axle.