I normally use 10# pounds of grain in a 7.5 gallon boil, and normally hit between 1.045 and 1.055 OG, after it evaporates down to 6+ gallons, depending on the amount of dark grains used. My stove setup cannot boil much more than 7.5 gallons.
If I were to add, say, 5# of grain but keep my water constant, should I expect a roughly 33% increase in my original gravity? Or, can water only hold so many sugars before it becomes saturated, and the only solution is to increase my boil volume?
If the latter is true, what is the theoretical maximum OG of preboil wort for a given amount of water?