I voted that I would be dealing significantly more damage. However, I should be clear about how I made that analysis.
I currently play an archer. Because I haven't played the character in some time, I have built up XP that would pay the difference in cost between profs / backstabs now and the future cost. So, except for body, effectively my stats wouldn't change.
However, I expect that monster body will decrease with this rollout. Thus, I feel that even though I would deal the same value in damage, my effective damage would be much higher, because the actual value matters much less than the percentage of health of opponent hit.
-MS