Some years back, for reasons I won't get into, I was flying a 767 and the fuel got 12,000 lb. out of balance. By the time we landed, we were still 10,000 pounds out. The ailerons were noticeably off center, but I was amazed that there were really no significant control problems. I think the usual limit of 1500 lb. or so is set just so they don't have to go to the trouble to flight test greater numbers.