Oh yeah, I never came back and fixed this.
So the easy formula for calculating how much horsepower a given weight reduction is "worth" (in terms of HP:Weight ratio for acceleration, obviously this doesn't take into account effects on handling and balance):
E = (W/R)*H
Where E is your new "effective" horsepower, W is the starting weight of the car, R is the reduced weight of the car, and H is your actual horsepower.
For example, take a set of numbers not unlike a stock 370Z on a DynoJet (3320 lbs, 275 rwhp), and drop 100 pounds:
E = (3320/3220)*275 = 283.54
So 100 pounds off of the stock car is going to make the same acceleration difference as adding 8.54 horsepower.
How much the car weighs and how much horsepower you actually have can have a big effect on the outcome of this formula, which is why there isn't a universal "X lbs is worth Y horsepower" number for all cars, or even for one car given all the mod variations.
Let's look at a hypothetical TT 370Z which is otherwise stock (with some added weight for the TT system, let's say 550rwhp and 3400 lbs), and see what happens when we drop 100 lbs there:
E = (3400/3300)*550 = 566.67
So 100 lbs off of that car is effectively worth 16.67 horsepower.
Either way, in both cases you're gaining about 3% effective horsepower numbers for acceleration purposes, because you've dropped roughly 3% of the car's body weight.
So a good rule of thumb to go by from all of this, is that it's all relative to your starting weight. Think of your reductions as percentages, and that's how much they'll help your acceleration. If removing 20 lbs is worth 0.66% weight reduction on your car, then it's going to make a 0.66% difference in acceleration (or effective horsepower if you prefer to think of it that way for comparing to bolt-on power adders).
Keep in mind, as I said at the top, that this says nothing about the handling effects of dropping weight, and it especially doesn't cover the special case of rotating masses in the front or rear wheels (or the driveline or either end of the crankshaft, etc), which have a more pronounced effect that I don't know how to calculate
