Ok sorry, I just quickly typed this on my smartphone while on the go, without having the whole thread in view. But I already had the impression that the thread was slowly turning into an academic discussion...
Theoretically, you can argue with the exchange rate, yes. However, for houses with more than 2 residential units, something like that would be prohibited, for example, and I am generally of the opinion that such regulations are not there for fun, even if this regulation with the 60-degree outlet temperature at the boiler applies not to single-family homes [EFHs], but only from 3 residential units onwards.
Everyone has to decide for themselves, but for me, a traditional boiler requires a minimum temperature of 60 degrees (and not significantly more because of lime scale) and 43-50 degrees for a hygiene storage tank. Hygiene storage tanks are usually somewhat larger, but they also have fewer thermal losses due to the lower temperature compared to a conventional boiler with 60 degrees.
On the topic then: The Rotex HPSU Compact 508 with 6 kW achieves an annual performance factor of 3.72 in heating mode and 3.77 in hot water mode (edit: calculated with an example at -14 degrees standard outside temperature, 35 degrees flow temperature, and other 'realistic' assumptions). At a heat pump tariff of 19 cents, you arrive at the 0.05 EUR I mentioned, slightly above at 20 cents. Of course, there are differences between summer and winter; on the other hand, there are also significantly more than 180 heating days and significantly fewer than 180 non-heating days. Standby losses are somewhere between 15 and 30 EUR per year and could not even be halved with a 200-liter tank, but they would be somewhat lower. Savings potential is maybe 10 EUR per year through a significantly smaller tank. Let it be 50 percent more – 15 EUR.
Because of 15 EUR, there is no need now to recommend a smaller storage tank. The tank is perfectly fine, has no noticeably higher operating costs than a 160-liter or 200-liter tank, and is absolutely hygienically top. In addition, the tank can be controlled at 50 degrees, which might be questionable with traditional tanks (i.e., I say it could be questionable and you have a different opinion).