I've been looking into the same topic for a couple weeks now. With my system, I do 5 gallon all-grain batches, but have to do two separate 2.5 gallon batches and combine them at the end to get 5 gallons. I was having a problem with an extreme maltiness in my brews. I first thought it was DMS or under-hopping, but eliminated any flaws in my process that could have made that possible. I finally read something about Charlie Papazian saying that any brew he tries that has had a boiloff greater than 15% has an odd over-malty flavor. It sounded a lot like the problem I was having. Doing 2.5 gallon batches, I was only boiling 3/4 of a gallon so I was going from 3.25 gallons down to 2.5 gallons. This amount of boiloff wouldn't be a problem for 5 gallons, but boiling that amount down to 2.5 gallons meant that I was boiling off 23%. I can personally attest that after I turned down the boil, I no longer have that problem. I was creating too many melanoidins and carmelization. All you need is to see movement or turning on the surface of your boil. The water doesn't have to be leaping out of the pot. I cut my boil volume in half and now just get a nice gental boil. Making very consistent brews now.