Well i do agree with trying to save weight, if all I did was flatwater, I would consider doing this for sure. It would not make since for the surf at all. A stock tank with a gallon of fuel will slosh all over were as that tank being way smaller will not slosh around till you have next to know gas. I thank a 2.5gallon tank would be perfect. I have a 5lb battery in my ski, and it works awsome, not sure how it would work on a high hp mod motor, and if you running TL it wouldnt be good probably either, but it runs my rule 500 bildge constantly, and I havnt had any problems. My ski sits perfect in the water, and feels great. I have the battery in the back corner of the stock tray closest to the middle of the boat with blocks of foam holding it in place. I am happy with the setup.
Me and a friend both noticed my ski feels alot lighter with a -7lb battery, and no reinforcement. I dont want to reinforce untill I have to. The weight is more of in issue for flatwater then it is in the surf. I may just build a square for the surf, and reiforce it like a tank. BR attemps on head high surf is hard on a ski, I am thinking about trying for a BF. Skis get fugged up when you try big shiot you have never done.