I saw an incident at New Smyrna Beach in '95 where a car got turned, and the nose went over and along the top of the front-straight wall and took out three or four poles of the catch-fence. The people in the first couple of rows in the bleachers probably got considerably more excitement than they wanted/expected on that restart. So this is something that has concerned me a bit, too.
I seem to recall some fences that go straight up - there is no angle out over the track near the top. So that's one suggestion - always have the catch fence angle out over the track to catch flying parts sooner.
Also, thicker (bigger diameter, thicker wall) catch fence poles, with heavy cable lines every 5-6 feet up the fence. Some tracks have cabling connecting the poles and acting as a containment (like the highway department has in medians to prevent cross-overs) between poles, but I think some do not. And a little distance between the wall and catch fence.
An impact-absorbing barrier could be helpful - doesn't have to be SAFER barrier (maybe steel plate, cables, and old tires?), but something to absorb a little energy before the car tries to launch over the wall.
I don't know what you can do about sprints/midgets getting airborne and potentially going over a wall. I've seen some very spectacular pictures of sprint cars well up in the air, and I suspect those started when one car wheel-launched over another. Perhaps some started when a car got turned toward the wall and already had some wheels off the ground when it hit, so the impact sent it end-over end and airborne? In that case, maybe an impact barrier might help.
NASCAR and/or the Indy cars have encountered most of this over the years (think about Bobby Allison's ride when he took down the catch fence at Talledega). Unfortunately, they mostly require renovations/modifications that are expensive for a local short-track.