Stock I believe is not difference but ECIS is
The only difference on the ECIS is the filter size. The 325 uses a 6 inch filter and 330 uses a 7 inch. I don't know why there is a difference in filter size. The old e36 325/328 used a 9 inch filter! From what I know, depending on the filter diameter, a 6 inch filter will support a car up to 280 HP and a 7 inch is like 340hp. The difference could be the 330 draws more air and requires a larger filter but again the e36 used a 9" so it makes no sense.