a couple of things to consider.
First, i looked at these today and all I can say is wow. What a deal, I would be very curious on the TDS (total disolved solids) reading on it, especially if it seems "fast". Don't get me wrong, I would order one right now if I had $100 laying around.
As far as the kalk to the auto top off... Kalkwasser is considered the best method of adding calcium by several, however there is no shortcut to adding it correctly. It needs to be dripped into the system at 1 drop per second or slower and done at night. Why, because it can cause pH fluctuation. At night your tank usually has more disolved CO2 gas, which causes pH to drop. Kalk raises pH, thta is why it is recommended to drip at night..... somebody correct me if I got that wrong. Anyway, I wouldn't add anything to the toppoff water.
I have been doing some research on RO/DI lately and have found several articles on it. What concerns me is that unit such as these, which take out everything in tap water, including chemicals designed to kill bacteria are removed. The water is then capable of becoming unhealthy because bacteria can multiply. I currently get my RO at my local Meijer from a Culligan machine that has a UV sterilizer on it. Of course that doesn't kill germs in my container, just on the water from the machine. I don't know if the holding container could harbor harmful bacteria or not, perhaps because there is no light no bacteria could flourish? I would love to get a under the sink unit with a tank so I could use it to drink too, but I'm not to sure about what I've read. I have seen additives from aquarium chemical companies to be added to RO/DI water... I wonder if these are meant to kill anything in it? Maybe someone has done some fact finding and could shed some light on this for us?