Charcoal canister change is recommend by Bavauto every 120,000-150,000 or so miles. I understand if it clogs it will reduce gas mileage. If clogged you might smell gas fumes. Also, if you are in the habit of topping off your gas tank, it can send raw gas and not fumes to the canister.
I replaced mine with no change in in gas mileage. Even so, you might want to recheck all the rubber hosing associated with the canister/blow-by/PCV like operation for vacumn leaks.
Thanks,
1995 525i Auto, M50TU 2.5L, EAT chip, 1/95 build, USA, 205/65/15 tires, ASC+T, HID, lumbar, EC Mirror, BMW Alpine 5 radio with BMW-Pioneer CD Changer, abt 236k miles, Oxford Green/Parchment