Having an intercooler will lower boosted air temps by 100-200 degrees F, depending on lots of factors. A water/alc injection system can as well. However, adding both won't double the cooling effectiveness. It's all about delta T. Water/alc won't make a significant difference when combined with an intercooler - for most 6.5 owners. I tried water/alc on an intercooled 6.5, and found adding water didn't make a measurable difference in EGT (constant load on a 6% grade, then switched water on/off).
An IC is the best solution. It's always there; no space is lost to a water/alc tank & plumbing, no need to replenish a consumable supply, and no worry about winter freeze-ups.
Competition diesels are benefited from having both, mostly due to big boost numbers.
Jim