I think its just a trick to sell not good gunners at good price.
First read this thread(see how guy nicknamed "slick" calculate TA):
http://wlnf.twowar.com/index.php?page=Thread&threadID=226&pageNo=4&highlight=sexy_neko
It should give you a hint how good they are at game mechanics.
UK acc gunners classed on time with resonable base hit more than 4000 ability with sailor crew 989. TA is easily more than 5m - sure much more above cap.If the mystery "overflow" of ability would exist many NFNA players would surely noticed that - but I never heard about it, and my pair of acc gunners(120 lvl classed on time 150v) shoot normal.
There is possibility that NFEU is different - i asked my experienced friends on EU.They said that its....lets say "not true" :D.
Kinluu is right - my experience is same.Hard to calculate ability cap(it needs some efforts and stuff) but it exist, works normal and its lower than reload cap.