I know I can feel the change in balance when I put a 5 lb platy of water in the front pocket on my packs, about 10" away from my back. But even with 20% of the pack wt located that far from the back, its still OK.
Many packs have a gap to keep the pack off the back. Exos, most heavy packs, internal and external frames. Only lightwt packs hug the back really, and they are the ones that least need to.
I wouldnt worry about it personally.