Sprinter trannies are known to have heat problems so a second cooler would be prudent. For this reason I've changed my fluid and filter sooner than the recommended distance in the manual. It really does not take a lot to burn the fluid in a sprinter. It can happen with just one day of spirited driving and not even much of a load (500lbs even). Definitely the transmissions in the sprinters were not designed for north american style driving but the more sedate euro driving styles.
One other point is that everyone should order a tranny dipstick as it is so hard to measure the right amount of fluid (2002-05 take 7.7L). So one other point is the difference between low and full on the dipstick is not a lot (less than half litre).
Rob