And from what I understand, as long as the system is allowed to flow through and not sit and stagnate, it would be fine to use on the same heater as the drinking water.
In MA there is a specific requirement on non-isolated combi systms to run the circulation pumps X-minutes out of every Y-hours to mitigate stagnation issues. (I don't recall the ratio)
For space efficient (and reasonably efficient) combi systems, this guy's reverse-indirect HW heater as buffer approach is about right, and has a high-capacity heat-exchanger:
http://www.heatpro.us/designtree/documents/tanklesssys.htm
The other advantage with the above system is that the tankless only heats (and the tank only stores) "dead" boiler water, cutting down on scaling & corrosion issues in both. It's not as efficient as a mod-con + indirect, but it's not bad- better in small heat-load scenarios than McMansions. If the domesting HW flow is detected with a flow switch to inhibit heating system calls the HW delivery has significant buffer to work down before the tankless needs to fire at full modulation- the best of both the tank/tankless worlds from a HW heating point of view.
Used as a boiler, an on-demand will run a few percent more efficient than the EF numbers imply since it'll have guaranteed longer duty cycles. eliminating efficiency-robbing short cycles like half-gallon handwashing draws. It generally takes a couple of minutes for even low-mass burners like a tankless to hit their efficiency strides. Draws of less than a gallon or so results in rather poor relative performance (under 60% efficiency), draws of a half gallon or can even be less than 50% efficient.
I tend to believe people who actually measure stuff: These folks did a comparison using a un-buffered Rinnai tankless (82EF) to heat HW & run an air-handler and it tested out in the mid-80s for combustion efficiency vs. high-70s for a pretty-good forced-draft tank heater in a similar system & load. It's shoulder & summer efficiency numbers were favorable by comparison as well.
http://dsp-psd.pwgsc.gc.ca/collection_2007/cmhc-schl/nh18-22/NH18-22-106-108E.pdf
If going combi-system, almost any tankless (buffered or otherwise) will likely outperform any conventional tank heater with some margin. But buffering the tankless with a reverse-indirect will squeeze a bit more performance out while removing all the tankless quirks in the process, and avoids any stagnation issues. Heatpro's combi probably beats the eKoComfort design's efficiency, during the heating season, and possibly even during the summer.