Will it be possible to stretch your stomach permanently? Not temporarily.
here is my thought process right now
in order for this upgrade to benefit you, you must stop eating for most of the day. however, this will usually cause you to miss your calorie requirement, so players would not want to do this anyway.
also, when you redeem the benefit of the increased hunger, the lost capacity will be regained. the only difference: the permanent stretching upgrade allows you to start your stream without having lost any capacity. but you can already do that with ghrelin for an extra $75.
so basically: permanent stretching upgrade = save 75$ per stream
but in the game's current state, i don't want to give the player more things to SAVE money on, i need to give them more things to SPEND money on