According to the first post;

#1 Ballista lvl7 archery has a range of 1890

#2 Archers Tower lvl4 archery has a range of 1560

ATs require lvl3 walls to begin so Lvl5 walls should not add 330+ to the range. Even assuming a 10% base raise for each lvl the total should be only 1820 (1300 X .2) + 1560. which would allow the ballistas to strike while still out of the range of the ATs.