Stephen,
Of course you are right. There is no direct correlation between weight and HP because every track is different. I chose 50hp as a wild guess simply based on local lap time comparisons. On a popular 1.8mi track here a keen driver (Grand Am driver Kevin Roush for example) might run 90 sec. in a bone stock 2.5 Box. He then steps in to a 250hp Box S and turns 88.5s. When he comes back to the pits our 2.5 Box has 100lbs removed and he proceeds to run 88.5s in that car. So on this day, at this track with these cars and this driver, 100lbs weight removed was roughly equal to 50hp. Switching over to the Auto Club Speedway in Fontana Kevin will be checking his email down the front straight at 130mph in the 2.5 car as Turbos and Vettes whizz by doing 165.
Feel free to use whatever comparisons you are comfortable with based on your own experience. This is just one crude example of the benefits of weight reduction on our cars. Your mileage may vary.
Here is a link to the 15lb Oddessy 680 battery I am using. If you get the battery also order the mounting cover they have. It is pretty slick:
http://www.odysseybatteries.com/battery/pc680series.htm