I doubt a static launch system would introduce enough energy to make the battery savings worth it. Most of the energy during flight is spent maintaining speed while climbing to altitude. The linear rail gives you a lot of speed, but zero altitude.

Still, there might be some weight gains that come from reducing the motor's peek power requirements, and those multiply across the entire flight.

I was wondering if it could worth implementing a two-stage system. The booster stage contains a lot of batteries, extra motors and a beefed up landing gear. The booster batteries contain enough power to get the aircraft up to it's cruising altitude before detaching and more or less gliding back to the departure airport. All the second stage would have to do is maintain cruising altitude for the rest of the flight then land.

They do something like that for aircraft carriers. But sailplanes use winches, which are a lot cheaper.