Nah, even Harry knows that Kincaid would respond 'oh, if you want it finished' and then Harry would look stupid :p
I think Harry would realize the risk that Kinkaid might just decide to finish it, so not do the tease in the first place. I think he has been convinced that monsterhood isn't an inevitable consequence of the WK-Mantle, so he plans to live until he figures out how to fix the problem.
But Kinkaid? I'm not so sure he'd be willing to take another whack. Recall their history on the topic:
1.
Kinkaid: "Never seen a wizard go all-out before."
Dresden: "You still haven't."
Kinkaid: <thoughtful pause>
2.
Kinkaid: "If I had to kill, I'd do it with a rifle from a thousand yards."
Dresden: <silent "
oh shit!">
3.
Kinkaid: <kills Dresden from 1000yd with a rifle>
Dresden: <comes back from the dead>
At this point, I think Kinkaid would be like, "I took my best shot. If that didn't work, I'm facing lower and lower odds of success. I don't like how that plays out. Not gonna take that contract." I think Kinkaid has his own personal code of honor, and would do his best if he somehow found himself obligated to do so... BUT I don't expect he'd be happy to do so, and would look for an exit-clause from that obligation.