The way I understand it, Delta-T is temperature differential.. so if the water in the DHW tank is say 50*, and the water circulating in the coil in the tank is 150*, then the Delta-T is 100*.
If the water is circulating slow enough in the coil that it enters at 150*, but returns to the boiler at 100*, then the Delta-T is variable, and the amount of heat transfered from the coil to the water is less from the cooler part of the coil. If the water leaving the coil returning to the boiler is the same temp as the DHW , then there is at least some of the coil not tranfering any heat into the DHW, this system need either faster circulation or hotter input temperature to make the coil more efficient.
If the water is circulating fast enough in the coil that the water temp is constant through the entire coil, then the Delta-T is constant, and the amount of heat transfered to the DHW is the same for the entire length of the coil... The heat transfer is at maximum.. this it difficult to achieve, because the flow rate would be very high in a coil.
This is an ongoing discussion in the automotive world too,, Some people believe that you must slow down the circulation of the coolant in a radiator to make the radiator more efficient.. But if the entire radiator surface is at the same maximum temperature, then the Delta-T is at maximum over the entire radiator surface,, shedding the maximum amount of heat into the ambient air. This is acheivable in an automotive radiator, because of the volume of water that can circulate through the tubes..