Tony - There is no heating coil. The 350lt of water sloshing around the store is the very same water pumped through the boiler, so coil absorbtion rates are irrelevant. Personally, and for the very reasons you've stated, I wouldn't want an indirect boiler circuit. (Apart from the solar coil).
Yes, on stores where the boiler heats the store directly the the coil adsorbtion is not relevant.
But many would not want the boiler directly heating the store water! See Dan's cylinder!
Tony