A question for Germans: As of right now, what would you say are Wowereit's chances of being the SPD's nominee for Chancellor percentage wise?
Uh, hard to say. 10%? 20% max.? Probably much closer to 10% after his decision to form a coalition with the CDU instead of one with the Greens in Berlin.
Gabriel, Steinmeier, and Steinbrück are the relevant potential candidates at the moment. Wowereit is probably fourth in line, maybe even fifth, if we count Olaf Scholz in.
And are there any polls showing how the SPD would fare if he were it's nominee?
We don't do such polls. There are only "which party would you vote for" and (hypothetical) "which Chancellor would you vote for" polls? I don't know if a Merkel vs. Wowereit poll was ever done though.
I know that, but there must be polls on how people's voting intentions would change depending on who a party was led by. I realize it's by party, which I think is a much better system than the one we have in the US.