I was worried about the same thing, but it has not been a bother to me. I prewet and have filled the tubes with the required water, and then the concentrated developer (so then active pouring time is just for 100cc's) (pour with the tube on a slant so a water bell doesn't slow things down). I have also I develop for 10 minutes and haven't seen any problems. I have also done a 600 cc water then 600 cc developer mix and nothing is different.
I am using 1050 cc water and am mixing 150 cc liquid developer in this.
Some time in the past a member posted his invention for filling while turning but I cant find it.