^ depends what year, my S211 had a 5 speed auto as it's the i6 engine not the v6 so it may have been an option as some point or standard on the newer E320's with the V6 engine.
Saying that mine seems to do 5mpg more than that if the newer model??
Just today I had 47 mpg on a run of 25 or so miles
the 270 will be more economical than the 320 Inline 6 eu3 engine.
The 320 also appeared in the W211 in an I6 with eu4 in late 2004 to mid 2005 before being replaced by the V6 engines. You can tell with the engine cover. The Eu3 engine has a black and silver cover , the eu4 engine has an all black cover.
The eu3 engines are more efficient than the eu4 ones.
I've had both the W211 E270 5 Cyl & E320 CDi IL6 and in real terms there was little difference between the two with regards MPG.
The 270 felt slightly better to drive round town, but the 320 was by far and away the better engine overall.
I would say on the combined cycle you would save 5 MPG maximum if you went to the 270. Although the biggest problem you will have with the 320 is resisting planting your foot at every opportunity....