The spray that gets in your face and all over your jacket comes off the top of the tire. Unfortunately, even just cutting off 3 or 4" from the front of the fender will allow it to get by. You need the whole thing if you want it to do the job it was designed for.
I cut off a few inches from the fender on my 550 because it was smashed up in the front, and I get sprayed anytime the road is wet. If you want to see how it looks, open the bike-pics link in my signature line. It's the black 1976 CB550K.