I don't see much weight added
I don't see much weight added by the battery, considering high performance car engines tend to be pretty heavy 500-600lb. Less than the battery but overall it won't be by much. They can also shave weight by going carbon fiber since it will most likely be limited production car.
Also, neither the Bolt nor the Leaf is indicated to hit over 250 miles. So far GM is saying max of 200 miles. Leaf based on their tests is estimated about 180 miles. There is also no indicator that they will put out those cars 18 month in advanced because so far latest indicators point to Bolt might even be pushed back to 2018. Now of course it is not impossible for them to push out the car earlier than Tesla just for the sake of publicity. But even then I don't see much prospects, specifically because based on LG ramp up times and how many different companies they are supplying, I can see nothing more than a few limited production model of each.