I haven't wingsuit BASEd (yet) but BASE 346 recently sent me some of the ProTrack data that went with his video from Kjerag. You can see from the graphs here and here that for the first 8-9 seconds and the first 800 feet or so, the vertical speed is building before it starts to decrease. It's difficult to tell how much forward speed there is without a GPS but it's not going to be as significant as when the lift is being generated and the vertical speed starts to drop.
So, in answer to your question, yes, 500ft probably is too low and you need at least a 1000ft to make it "worth it". (Unless you are a crazy, super-tracking Norwegian, maybe...)
If you look at the BASE Take It Too The Streets film at the skydivingmovies ftp, you'll see a birdman jumping from the tower(s). His speed doesn't differ that much in the beginning, but he does have way more clearing from the tower straight from the start. His forward motion is much more efficient (or at least the video makes it look that way)