Thanks for the reply - since posting we've done some interesting work in profiling the water across locations and determining the influence of each locations' water on espresso extractions by controlling for the usual grinder and coffee variation (same coffee/grinder) and pulling shots with the same temperature, pressure and a range of normal brew ratios.
Overall we had confirmation of much of what the water quality handbook has to say, with some specific data on optimal pH and alkalinity.
We found the coffee to be most sweet, balanced, nuanced and syrupy with a pH below 7.9 and significantly improving as the pH dropped towards 7.6. Above 7.9 yielded very thin, dull flavoured shots with increasing sourness as pH creeped up towards 8.3 at the highest.
Alkalnity (KH) also had a tremendous impact on the extraction. The shops with higher KH around 35 mg/l demonstrated more balanced and pleasant acidity as well as an overall better extraction with respect to sweetness, flavour nuance and texture. With a KH lower than this the acidity became progressively more aggressive and the overall extraction suffered as well.
Of course, these two variables can either complement each other if they are both ideal (pH towards 7.5 and KH towards 40), mitigate each other if one is not ideal, or they can completely preclude the possibility of getting great coffee if they are both off-ideal.
For instance, comparing two shops with a pH of 8.1 where one shop had a KH of 35mg/l and one had a KH of 25 mg/l, we found the 25mg/l shots to be of greatly inferior quality (likely scoring a 1.5 to 2 on a WBC taste balance scale) while the 35mg/l shots were probably more like 3.5. Again, this is with the same coffee, grinder and with many other extraction variables controlled as well as we could. I should note that TDS was also constant at 90ppm here and we saw no significant differences in other mineral contents.
Another interesting comparison is the performance between our two best shops. With one at pH=7.9, KH=30mg/l and TDS=100ppm and the other at pH=7.7, KH=30mg/l, TDS=90ppm, we found a significant difference in overall quality. The first shop (pH =7.9) had slightly less aromatic nuance both in espresso and in a 12oz latte. I would say the shop with a pH of 7.7 would likely have scored a taste balance of 4 - 4.5 while at 7.9 it would be more like 3.5 - 4.
All this is to say that I also am becoming a great fan of water formulation technology and am viewing it is an absolute necessity for our sort of business (attemping awesome coffee across 11 locations and growing...) given the less-than-deal and varying nature of our water.
Also, I'm looking forward to the day when espresso machines come with a built in "equalizer" that allows the barista to twerak the calcium, pH, KH, etc shot-to-shot!!