i would say that the coil is at the top for the sole purpose of quick recovery of hot water if all the existing hot water should be drawn out of the tank at one time just fire up the coal range and presto half an hour to an hour later and you should be able to draw of hot water again...to me this is the logical reason why the coil is higher in the cylinder...and as the water sits in the cylinder it forms strata layers based on relitave temprature and density of the water...which is the reason the draw off point for hot water is at the top of the cylinder and not in the middle or the bottom, so the heated water at the top will not be circulating as you suspect this will only happen when you open a hot water tap introducing cold water into the cylinder causing disturbance and mixing up the strata layers.
You heat storage capacity would be more affected by poor insulation of the cylinder and flow and return lines of the coal range heating coil so if your worrying about it then get a cylinder wrap which should give you a more efficent heat/energy/money storage check if your pipes are lagged at least intill they dissapear into the wall as this will also help prevent heat loss.

good luck.