5.2 just adds a bunch of sodium to the water and doesn't really improve brewing in most cases. But in this brewer's case, starting with RO water could actually get a benefit since 5.2 adds a bit of buffering that keeps the pH from dropping too low when the calcium level is increased. The target sulfate level in brewing water is highly subjective and up to the brewer's taste. Bringing the sulfate level to 120 ppm may be a good start, but I find that boosting it to around 300 ppm is pleasing in an IPA.
Since gypsum can be added directly to the boil kettle and would not be needed in the mash because RO water is used, I recommend that 5.2 be deleted from the mashing water and add most of the gypsum directly to the kettle with a little added to the tun to help reduce beerstone formation. The calcium reacts with oxalate in the mash to reduce beerstone potential.
As AJ Delange likes to say, 5.2 works great for brewers that don't check their mash pH and doesn't work at all for brewers that do check pH. In other words, its snake oil.
