I’m not sure how the numbers work out, but I would guess that the weight lost by the stripped interior is not too far off the gain from the addition of a cage/fire suppression/etc., so it might work out in the end. In ‘production car’ classes it might be interesting to impose a minimum weight not less that the factory…

Sea landing will be necessary for higher energy launches (heavier payloads and higher orbits or escape trajectories), but if it’s a lower-energy launch like a smaller satellite into LEO then there will be enough fuel to return to the launch site, making things a lot simpler.