I generally start out with 2.5 l/kg. After 20 min or so, I add another 0.5 l/kg (I have a poorly insultated mash tun), and hold for 40 min. The brewpub I worked in always mashed at 3 l/kg, and the brewer claimed that that was best especially for British malts. It works well for me.
Now, the question about whether or not a thicker mash ends up with a more fermentable wort, I always get confused about that, but I think that thicker = more fermentable. But, temperature and time are just as important (or even more important). If you want more fermentable wort, mash at 149-151F for 1 hour or more. While most of the starches will be converted to sugars in a much shorter time, the sugars will continue breaking down from unfermentable to fermentable the more time you give it. (Rumor has it that the major breweries mash for 3-4 hours with their lite beers to help make them more fermentable.)