You have to boil down pretty far to get any flavor contribution. Not quite to candi syrup level, but much further than a to-be-diluted wort. A common practice in all-grain recipes for wee heavies and such is to set aside some of the first runnings and boil off 3/4 of the water. I'm assuming you won't be going to that extent. A small amount of dark crystal (C-120 or Special B, for example) can mimic the caramelization you'd get from an extreme boil.
However, non-hoppy beers are a good candidate for partial boils, as hop utilization is directly related to the amount of liquid. Probably not the answer you wanted, but you can be happy that you can save time and money with a concentrated boil and cool quickly by adding ice.