I usually carry one liter in each side pocket and the rest as high and forward as I can. Recently I carried a total of 6 liters on a trip. In the pack I used a 2L bottle and two 1 Liter bottles so it would be easier to pack.
My total pack weight was 25 lbs, with a little over 13 lbs of water. The pack was a Mariposa Plus. So the pack handled the weight well. It was just a little heavier than I wanted it to be, and of course got lighter everyday.
I have never really liked hydration systems for a variety of reasons. But I am going to try a 3L bladder, just from the easier packing point of view. I often do hikes with my Murmur carrying 4 liters of water, but a base weight of 4 - 5 lbs. Now we talking about a pack weight usually in the 17 lb range, which is a little uncomfortable in a Murmur, but on these trips, it is usually just for one day, and at the end of the day I am nearly out of water, and have a reliable water sources along the trail for the next few days, with the exception of my route back which requires starting out with 4 liters.
I all of the above situations, I just want to make sure the water is evenly distributed from side to side, and no more than two lites on each side. I did try two liters in the side pockets of the Mariposa. A 2L Platy in the big side pocket, and two 1L in the other (had to put one in upside down), and this did make the pack feel ackward.
Ocassionally I do trips that require 2 - 3 gallons of water, and I just switch to my Gregory Whitney 95 internal frame or my Kelty Serac external. But to be honest, I am just shying away from these trips anymore, because the UL hiking is just plain more fun. And I am finding that I travel much further, requiring less water with the UL set up. So I can do some of those 2 gallon trips with 6 liters or even less.