There is no "leaching of metals by the soft water". You only need to provide sufficient calcium hardness (for calcium ions) and total alkalinity and appropriate pH (for carbonate ions) to saturate the water with calcium carbonate when you have plaster/grout surfaces that contain calcium carbonate. For a spa that is acrylic, there is no need for saturation of the water with calcium carbonate. The same is true for most vinyl pools.
The idea that saturating the water with calcium carbonate will prevent metal corrosion is very dicey and controversial. See
this discussion among corrosion experts. Basically, it is very difficult to coat a fine layer of calcium carbonate to protect metal especially when flow rates and temperatures are changing. Metal corrosion is much more dependent on pH, dissolved oxygen, and the level of other oxidizers. Some metals such as stainless steel are also affected by the chloride level that interferes with reformation of their oxide passivity layer.
Many municipal water supplies have low calcium hardness (CH). My water supply has 55 ppm CH and the other water parameters combined have a calcite saturation index of -0.6 yet the copper pipes in all of the serviced homes are not corroding. There are 300-500 ppb phosphates added to the water as a corrosion inhibitor and is far more effective then trying to saturate the water with calcium carbonate.
So the reason the advice for CH levels is inconsistent is that the advice to have a higher CH in the non-ACE spas is wrong (in terms of metal corrosion). There really isn't any inconsistency -- there's just wrong advice. Now, that said, one can reduce foaming in the spa with a higher CH of around 120-150 ppm but that would be incompatible with the ACE system since it needs a very low CH to prevent calcium carbonate buildup on its very sensitive plate material.
