7 dKh here. Better colors for acros and its closer to NSW. I also carbon dose with biopellets so keeping it under 8 dKh prevents tip burn.
Regarding drop in dKh, RR-Man is correct in advising a higher alk level will give you more room before you hit a critically low alk level. I've found the quick drop is the main killer, not having too low of alk. My calcium reactor has run out of CO2 before and by the time I switched out the cylinder, my alk dropped from 7.0 to 5.5. I had some issues getting my regulator on the cylinder and it stayed at 5.5 dKh for a day out two with no issues to the tank. My experience is a quick drop or increase will kill a frag but if you have healthy, stable colonies, they will be fine.
Also, the low alk (5.5 dKh) will stop growth but it didn't kill any acros in my system. Kind of a self buffer of slowing or halting growth when limited alk was available.