yep. water transfers heat about 2.7 times better than coolant. but it does not have the freezing, boil over, lubrication, rust inhibitor, etc protection that coolant does. when running more water, it is real important to make sure the system holds pressure (16 psi will raise the boiling point roughly 50*F - cant be more precise as it depends upon the ratio used).
i start out at around 50/50 and add water for the most part, not minding if i widdle down to 75/25 (water/coolant, respectively) after some time. it is important, IMHO, to use additives (lubricants and rust inhibitors) if a small amount of coolant is used (mostly water).
the gains from more water are not huge, and if one needs to run more water for a slight gain, there are other issues which should be addressed.