In case it's of use to anyone, I bought a long back bergen years ago to use as a standalone pack. I chose a long back because I'm 6' 3". More recently I bought some PLCE webbing and thought the two would work together quite well, largely because the kidney pads on the bergen are higher than you'd find on a civvy pack, but I was wrong.
The webbing belt sits right around my belly button, because of the length of the straps on the main yoke, and at that position the long back bergen sits too high to be comfortable. Also, too much of the packs weight rests on the webbing pouches and when it's loaded it's really uncomfortable.
Last week I bought a short back bergen to see if that was any better. At first I was put off by just how high the short back sat on my body, but after a bit of adjustment, of both the webbing and the bergen, it's actually turned out to be a pretty comfortable combo.
Even at my height, if I want to use the bergen with webbing, the short back is the favourite. However, if I need a bergen to use without webbing the long back wins.