Ok, i'm no engineer, but i have to weigh in here.
It seems to me this is a pretty good way to increase the efficiency of a modcon boiler. Obviously the payoff is questionable - as with all high efficiency appliances. Low return temps are the holy grail of modcon efficciency - going from greater than 140 (non-condensing) down to temps in the lower 100's can create efficiency gains of around 10% if i'm not mistaken.
it is likely that many modcon boilers on baseboard systems hardly ever condense unless extra baseboard has been added to the house to facilitate lower supply temps. Furthermore, when these boilers are operating at 100% fire with a target temp of 180 in prioritized dhw mode, their efficiency is lower than in space heat mode. In areas like mine, with incoming groundwater temps of around 36F, return water would be cooled substantially with a preheat tank, for quite a while. This would be especially beneficial in the morning. Cold outdoor temps would have the boiler operating at the top of the reset curve, condensing very little. With the preheat tank the boiler would be heating dhw gradually in a more efficient low fire setting, rather then in high fire dhw mode. Then later in the day if it warmed up, the boiler would not even need to fire if a system sensor is utilized.
I have a modcon/indirect with low temp radiant, so this would not be as beneficial for me as return temps are already pretty low. What I would like to try is a dual coil solar dhw preheat tank. Solar exchanger water in the bottom coil, boiler return water in the top. In times of high solar gain, the hot dhw in the tank would heat the boiler return water, eliminating the need for the boiler to fire. When there was no solar gain, the dhw tank would cool the return water temp to the boiler. Haven't found anyone who has tried this yet.