WN agree with your assesment. Fays environment was somewhat different.Here is a time sensitive link to what will keep Gustav moving nw to wnw for a couple days- textbook TUTT .Fay did not have this constraint. Although depending on storm relative position these can have either a positive or negative effect on development. You can see the outflow to the NE of Gustav slamming into it. I'm thinking it is having a neutral effect on development right now but will keep Gustav on a more wnw track.
http://www.ssd.noaa.gov/goes/east/watl/loop-avn.html