The title says it all but let me only clarify what topic I raise. Firstly, it's not a discussion about the direct veracity of their claims, such disputes were more fit to the time before the election. I fully expect you to digress, but I'd rather we didn't, because the question is "What Impact Did the SBVT Allegation and Advertisements Have On the Election's Outcome?"
The fact that some say they lied in every case, some say in no case, some say in certain cases, some say less than Kerry... is itself data regarding their impact. If EVERYONE thought they told no lies, then this implies something about their impact, regardless of the fact that everyone, in that case, might be mistaken.
Most of us probably knew the allegations were coming before we ever heard of John O'Neill because it was evident that Kerry's role in the Vietnam Vets Against the War and his appearance before the US Senate were likely to be brought up because they are, in fact, key points not just of Kerry's past but sore points in American History itself. That John O'Neill played his role might have not been surprising, and added a poetic circle in which John (O) and John (K) relived a debate they had back in the 60s. In fact, they literally debated on the Dick Cavett show (I wanted to give a link to the C-SPAN archives, where I watched it, but it seems to have been removed).
At the time I felt the impact would not be too severe for Kerry, thinking this all was well known enough to not shift voters, and naively hoping that Vietnam was in the past. I think history showed me wrong, but the evidence is not conclusive. However, I do think it hurt Kerry and is one of many things that can be said to have lost the election (that is, there are numerous things that can be said to have swayed a few million votes and any of them were therefore enough to "lose the election"). I say this because when the SBVT started advertising Unfit for Command and running other ads in earnest in August 2004, Kerry had come to enjoy a 5-10 point lead in the polls. During the onslaught from SBVT he fell into the dead heat stasis that persisted, poll-wise, until election day.
I don't think this hinged on the disputed facts so much as maybe the basic fact that people didn't know, didn't remember that Kerry was leader of the VVAW, people didn't remember the anti-war movement and by being shown snippets of his testimony and reacquainted with Kerry's significant role against the war, it seemed almost as if Kerry was solely responsible for the negative accounting of the Vietnam War which still persists in the American psyche. While I suspect most people did not hold Kerry's protesting years against him, quite the contrary, I think the current Wartime Condition of the US did not cross well with their intentions to pursue the War on Terror according to a Bush Doctrine.
Again, I offer my own answer simply to not be overly coy, to be honest about my ideas (and to display rather than hide any bias I may suffer), but the real point is the question and your analysis, Did the SBVT Campaign Effect the Final Outcome of the US Election?