My semi-educated guess is that it's about the same. The 25L capacity is the same, although the shape is slightly less irregular than the OEMs. I never had the OEM soft bags, but I did ask to see the inside of one fairly recently so I have a slightly fuzzy basis for comparison. I would guess the WL900's might have slightly less drag, but I don't think that I would personally go to the trouble of converting the racks and getting WL900's if I already had the OEM bags unless you wanted the zipper-lock for a little bit (not a lot) of extra security. I'm very happy with them, though, and I like that I can put them on an Experia if/when I ever get one.