Sounds like a pretty good way to reduce the size of basins.....my fill sinks function has the same twist as yours, coincidentally. Also, I use a stupid "fill landlocked oceans" function before filling sinks which sets sea areas that cannot reach the edge of the map to the lowest non sea level height of its neighbour squares. Which probably is adding to my problems actually, but at least everywhere doesn't have lots of little "black sea" pits for all the rivers to flow into.