Good points. The degree of suffering is indeed hard to measure, and in some sense actually impossible as u mention. When I frame the question in terms of morality Im not only asking which might be objectively worse in terms of absolute suffering, that of a human or a dog, but also to consider what potential justifications or rational goes through the mind of the agent responsible for the deeds, and what degree of legitimacy they hold.
As per you fruitful examples, its clear that suffering can vary drastically between different creatures, even ones of the same species. Simple nociceptive pain amongst similarly developed creatures will likely be fundamentally the same if slightly varied in severity on some normal distribution. The greater variation comes from the modulating effect of a psychological overlay, which is highly specified to the individual. All of which you pointed out.
My concern would be that in creating a hierarchical list of potential suffering, although on the face of it sensible, Im not sure it lends itself perfectly to a means of acting in a moral way. Clearly you can be very wrong in what you anticipate, and to rely on such a thing rigidly seems problematic. I mean on one side, even though the monk suffers less, does that somehow permit someone to subject them to more harm than they do another? I dont think so. So even in situations where the moral agent is actually correct in his anticipation of potential suffering, it still seems wrong to me that such action be guided by this sort of hard calculus.
Also due to the consequences involved, I think one should be skewed into anticipating the creatures suffering to be worse rather than milder. To overestimate what ones believes to be the resultant suffering of an action seems better than to underestimate when you consider the potential outcomes.
I would therefore agree with you regarding the theory of how objective suffering might well vary, but with respects to informing action relating to suffering I would air on the side of caution, such that if I can detect that a creature can suffer full stop, they deserve the same maximum respect from me.
I always worry about the slippery slope of considering the suffering of other creatures to be less so on some scale (esp considering how difficult it actually is to anticipate accurately the suffering of an individual as you pointed out), and with so much animal mistreatment in the world, it seems that much evil can occur from not allowing adequate range to the respect offered, and to horribly underestimating the reality.
Alex
(Ps sorry for the delay, I was crazy busy yesterday.)
good question. Lets just stick with human for now. 
