My tonneau is "missing a slat", and has at least 2 fewer than yours in the picture, but does not leak. All of the water came in from the "applique seal" between the plastic bit and the glass roof. Once that was fixed, no water!
Efficency doesn't really say anything about your battery -- it's how much energy you used per mile as compared to the "rated" 391Wh/mi (or thereabouts).
Are you going places without chargers? I usually run at as close to the bottom of the pack as comfortable, arriving to SCs with 8-12%, and...
I've done over 5000mi now since I got my CT in the last week of December. Average is 514wh/mi, that's with mostly highway driving at ~66-67mph in a 60, or 75-78mph in a 70mph zone around WA and OR. A/T Tires, wheel covers don't seem to make a big diff. Some off-road driving, snow, gravel, hills...