The limit of this is of course the line at which parameters kill corals. Which seems to be somewhere just above 12.5 ALK. I ran my frag tank and main tank and 12 ALK for over a year and my experience was it did result in more growth than I currently see, but at a lot more risk. At high levels of alk and calc a small dosing error can become a big issue. Growth was fast, but I felt the corals were more "on the edge". I believe I experienced more STNing and RTNing than I currently do with a more moderate level of 9.5. I also experimented with a frag tank at a 6 ALk and actually found colors to be enhanced, but it was a lot more risk as well.