Ok so I think your point here is that a lamb could react the same way to being in intense pain (by moving away) as it would react to low pain stimuli, and so its hard to tell if they're really in pain or just a bit bothered? By this logic, you technically can't determine they're pain free either as it could be one or the other. One way to show if this is the case is to inflict painful stimuli on some lambs and merely handle others as a control. They would then supposedly show the same 'escape' response if you put say a balloon over their testicles or just handled them, as with using a rubber ring. A study like this was done (see links) and the lambs that were only handled showed very different behaviour to that of the castrated lambs (this was based on shepherd observations by the way, not the researchers, to ensure they didn't 'see what they were looking for'). Heart rate stayed high for a number of hours in some lambs, blood cortisol was high and posture changed. What do you think about this ? Can I ask if you have ever noted lambs looking sleepy/sleeping in the hours after castration?The anatomy's interpretation of sensation before the brain will be virtually instantaneous.
It is then for the brain to assimilate all the other information to formulate a response.
Extreme pain results in immediate action which will often be a movement to reduce the pain.
Sensations are constantly being sensed and ignored. You are probably sitting down reading this and oblivious to the feeling of you buttocks being pressed against the chair until I've pointed it out [*immediate butt shuffle as a result].
Tickling has exactly the same reaction profile as extreme pain. It is a horrible pleasure.
Ultimately it all comes down to triage.
How much is the suffering
How long will the suffering last
What is the potential benefit.
If you are convinced there is no suffering, the other 2 points are superflous.
Is a very minor discomfort for 30 minutes, better or worse than a very brief pain?
If there is no benefit, don't risk any suffering.
I think there is no doubt that wool bearing breeding stock should be tailed. Many years of benefit and the need to be able to easily see any lambing problems makes it a no brainer.
Lambs that will be finished and sold before May will probably not need any rings.