The primary reason that I suggested a higher pH and a lower TA was to reduce the amount of carbon dioxide in the water. The amount of carbon dioxide in excess of that which is in equilibrium with the air can be reduced by about 50 %, and should reduce the corresponding carbon dioxide loss rate and consequent pH rise by about as much. Based on the particulars of this pool, I felt that a lower level of carbon dioxide would be especially beneficial.
1) The pool is heated. Warmer water will not hold as much carbon dioxide as cooler water.
2) Two blowers, two bubblers, two waterfalls, eight jets and the SWG will create aeration substantially in excess of most pools.
The reason that I suggested adding borates is that using an SWG in a plaster pool is especially challenging due to the competing need to maintain the CSI high enough to protect the plaster and low enough to prevent excess scaling in the salt cell. Borates are particularly good at buffering pH from increasing. 50 ppm borates (at a pH of 7.8) have the same pH buffering capacity on the upside as 221.6 ppm of TA have at buffering the pH on the downside. The boric acid sequesters the hydroxides created in the cell and prevents them from converting bicarbonate into carbonate.
B(OH)3 + OH- <> B(OH)4- (pKa = 9.14)
Also, keeping the CSI as close to 0.0 as possible without scaling the cell will reduce pH rise due to the new plaster.
[edit]When one is going to keep the CSI slightly negative in a plaster pool to prevent scaling in a salt cell, or for any other reason, then the plaster can be at some risk for etching and premature dissolution. In these cases, a higher pH will help reduce the risks, because a pH of 7.5 has about twice as many hydrogen ions as a pH of 7.8.
It is the hydrogen ions that react with the calcium carbonate in the plaster to form soluble calcium bicarbonate. This reaction causes the pH to rise. Maintaining a higher pH will reduce the amount of pH rise due to this reaction.
Maintaining a neutral to positive CSI is important to achieve maximum longevity of the plaster. [end edit]