Having spent a few years flying commercial 4-6 seaters at 120kts or so and then turboprops at 200kts and now 737's, it's been my experience that no matter how slow you are going, relying on the old eyeball alone is a factor of pure chance. The thing that makes the big difference and doesn't seem to happen that often in certain areas OTCA is everyone having a radio and being on the same frequency. If you are doing 330kts or 100kts and don't know where to look, then it's going to be hard to see!! There's nothing magical about 250kts. It's just a starting point.
Everyone should have an operating transponder and radio.