i have been told that you need to boil with lid off because chlorine and some corn tasting chemicals need to escape.
also, i do all grain and have a hard time getting 1 gallon per hour to evaporate as expected. it could be because my pot is only 12" diameter, i usually have a hops bag floating and taking up some of the surface area, and my turkey fryer burner is running at 100% capacity (could use a little more heat).