Yes, that thickness will not affect flow much, but we're talking theory and trying to squeeze every last bit of power out that we can....
Not to mention that the inside of the tube is a much harsher environment for the coating to live in compared to the outside of the tube.... Coatings have a hard enough time staying on the outside of the tube as it is, why make it harder on the coating by putting it on the inside????
Do they put the paper wrapper on the inside of the straw that contacts your drink????
