salaam,
many of us speak regularly about halal-ness of meat, as of 3rd March 2014, no meat slaughtered mechanically will be accredited by HFA:
http://halalfoodauthority.com/about-us/news/63-hfa-abandons-mechanical-slaughtering
logically this means that even if chickens (exclude red meat as they cannot die via stunning) do die from stunning, then at the point of slaughter the slaughterman would realise and discard that chicken.
I rang the assistant chief exec of HFA yesterday too and he guaranteed that the people who slaughter are all muslim, all check for signs of life at the point of slaughter, individually recite the shahadah (well, 2 thirds of it anyway ;) ) and that after careful consideration and discussion they have decided against mechanical slaughtering.
He also said that employees are trained well, and regularly monitored and reviewed, and that the only way it can not be slaughtered correctly is if the worker himself chooses to be careless/ignorant - Allahu a'lam.
Anyway just thought I'd share info.
iltemase dua