I've dabbled with water treatment but I'm no scientist and I've reached no conclusions.
I've tried putting my local water details into online calculators like the one on this site and these indicated that gypsum was the main requirement for pale ales and calcium chloride for stouts/porters. The amounts of other additions were negligible, but I did add them a couple of times.
I generally add a teaspoon of gypsum to pale ales nowadays, half tsp in the mash and half tsp in the boil, for 10 litre brews. For stouts its different, I put a tsp of calcium chloride in the boil for 10 litres. I steep dark grains separately from the mash, as they can affect the PH of the mash and someone on a forum suggested a separate steep. I have never tested for PH though!
My beer is good but there's room for improvement, and I guess yeast/fermentation and water treatment are the main areas I haven't pushed at much yet. I think yeast experimentation and fermentation control are much more important and worthy of research etc. I can't see me digging any deeper into water treatment.What I do is simple, easy to remember, and corrects the main issues, I hope! I almost exclusively make pale ales and porters/stouts.
My tap water is great to drink. It's very soft, low in minerals, so I need to add rather than take away.
If you're interested, get your local water report, should be online, and stick the figures in the calculator on here. It's pretty easy actually.
This article gives a nice brief overview of water treatment...
http://www.brewersfriend.com/mash-chemistry-and-brewing-water-calculator/