Death by exsanguination not painful?
This is of course a controversial topic with there being supporters for both methods of slaughter. First of all, The research paper that you have mentioned is a highly cited one and it is the foremost study that supporters of exanguination use to support their method of slaughter. However, the author himself (Wilhelm Schulze) has stated and I quote 


  "objective results presented for the captive bolt application in sheep
  indicates that the captive bolt device used is suspect" and that these
  initial "scientific findings and the results presented are only a very
  first contribution" and that they "need to be followed as a high
  priority by further investigations in the continuation of the
  scientific clarification of the issues of loss of pain and
  consciousness during slaughter of this kind with and without stunning
  using the same experimental approach with a representative number of
  grown cows of various breeds." (reference)


Since stunning methods have advanced (reference) from the time this paper was published (1978), it would probably be a bit unfair to hold onto this paper as proof for supporting exsanguination. Moreover, in another study, in which measurements were done with an electro-corticogram it was found that


  Captive bolt stunning followed by sticking one minute later resulted
  in immediate and irreversible loss of evoked responses after the stun.
  Spontaneous cortical activity was lost before sticking in three
  animals, and in an average of 10 seconds after sticking in the
  remaining five animals. The duration of brain function after shechita
  was very variable, and particularly contrasted with captive bolt
  stunning with respect to the effects on evoked responses. These were
  lost between 20 and 126 seconds (means of 77 seconds for somatosensory
  and 55 seconds for visual evoked responses) and spontaneous activity
  was lost between 19 and 113 seconds (mean 75 seconds) after slaughter (reference).


The two questions that have to be answered in exsanguination are

-- Does the neck cut cause pain?

-- How long does it take for the animal to lose enough blood for insensibility to occur and for any perception of pain or suffering associated with slaughter to disappear?

With regard to pain, in the process of cutting the major blood vessels leading to and from the brain a number of other tissues are also severed by the extensive neck cut including muscle, skin, sensory and motor nerves etc. There is a chance that this might cause significant shock and fear to the animal. The time to lose consciousness is highly variable in  exsanguination. (reference)

Very good arguments are presented in this article and I highly recommend that you go through it.