Maybe, but I think one can only really test if customers are willing to pay for battery-life if there are two otherwise identical models with different battery

Maybe, but I think one can only really test if customers are willing to pay for battery-life if there are two otherwise identical models with different battery life available. Otherwise it's the thinner model with all the latest engineering built into it vs a high-battery life model that was implemented by a low-resource team because it's an experiment. (do you want a iphone 6 or iphone 6 w/ extra battery life...)