Point 1: See the Are Borates Safe? thread for more details. The EPA limit for Borates is around 50 ppm due to the dog studies, a margin of error (MOE) of 100 and presumed small amounts of ingested pool water. It's all very conservative, but maximum label dosings would be no more than 50 ppm these days.

Point 2: The following table shows the equilibrium carbonate alkalinity vs. pH where the amount of carbon dioxide in the water is in equilibrium with that in air.

pH CarbAlk
--- ----------
7.0 ..... 3
7.1 ..... 4
7.2 ..... 5
7.3 ..... 6
7.4 ..... 7
7.5 ..... 9
7.6 ... 11
7.7 ... 14
7.8 ... 18
7.9 ... 23
8.0 ... 29
8.1 ... 37
8.2 ... 46
8.3 ... 59
8.4 ... 76
8.5 ... 98
8.6 . 127
8.7 . 165
8.8 . 216

However, the rate of carbon dioxide outgassing varies as the square of the CarbAlk so in practice the lower TA significantly reduces the rate of pH rise and does not need to be as low as the above equilibrium values. As waterbear noted, in practice a TA of 50 ppm is usually pretty stable in spas if 50 ppm Borates are also used (and along with 30 ppm CYA the CarbAlk will be around 30 with the pH at 7.7 to 7.8). However, if you've got heavy use of aeration jets in a spa, then an even lower TA may be needed.

Point 3: I disagree with waterbear on this one. I wouldn't say that there is any tendency of the borate buffer system to move the pH at all. It is the carbonate buffer system that moves the pH upward based on the out-of-equilibrium situation described in Point 2 above due to carbon dioxide outgassing. What waterbear is referring to in terms of up and down are the pH positions of maximum buffer strength (aka buffer capacity though it isn't really a capacity) which for the carbonic acid (really mostly aqueous carbon dioxide) / bicarbonate buffer system is at a pH of 6.3 while for the borate buffer system is at a pH of 9.15. So the carbonate buffer system gets stronger as the pH gets lower so it is great at preventing the pH from getting too low, at least until the TA is near exhausted. Likewise, the borate buffer system gets stronger as the pH gets higher so it is great at preventing the pH from getting too high.