Let me preface this post - I am not a water or chemistry expert, and I have not read the entire thread in great detail. However, I submit the following:
Greg Noonan (the mentor for Mr. Kimmich of Alchemist) notes in a BYO article that he adds calcium sulfate to bring the BREWING LIQUOR for IPAs up to 750 mg/L of hardness. See http://byo.com/scottish-ale/item/310...-from-the-pros
. This is consistent with the Heady Topper brew sheet screenshot taken from the 1000th brew clip on youtube, showing roughly 750 Hardness for the brewing liquor.
Using "Brewers Friend" mash chemistry and brewing water calculator, a water profile of Ca 127, Mg 6, Na 10, Cl 43, So4 268 (a relatively standard IPA water profile) for the entire water amount, with ALL THE SALTS being added into the mash, gives a total hardness (GH) of 763 and an Alkalinity of 51 for the BREWING LIQUOR (i.e. Mash Water). This is consistent with the 750 hardness figure. Further, Using the Grist from the Heady Topper clone on HBT (version 4), this estimates a mash pH of 5.4 with no acid addition.
I also note the El Jefe (India Dark Ale) clone recipe provided by the Alchemist in Mitch Steele's IPA book, says the water should have a total hardness of 400 ppm and alkalinity of 50 ppm. If you take the above profile (Ca 127, Mg 6, Na 10, Cl 43, So4 268), and apply that to the entire water amount (mash and sparge), you would get a total hardness of approximately 400 and and alkalinity of 50.
Conclusion: is it possible the 750 hardness figure is relating to the brewing liquor only, and provided you use RO water for sparging, you would end up with a typical IPA water profile?