Tests done on an F3D pylon racer showed a gain of 2000 revs at 176MPH. But there's more involved with normal sports type engines than just the prop unloading. Almost all of them are run well below peak HP so when the prop unloads the engine develops more HP to drive the prop even faster. This isn't a continuous cycle of course because the drag of the model builds up very quickly to balance any extra thrust developed. But another effect comes into play as engine revs increase. The airflow through the carb rises which means the depression (ability to draw fuel) increases. This should go a long way to counteract any tendency to lean out.