thanks for your question, to enable the reposition, jquery will go through the DOM tree and find the right position where the comment form need to be placed. so technically, it may take less than 1ms to do this job. this job is skipped when you disable it. so in fact, technically, it is a better performance. :)
the reasons I recommend to reposition the comment form is because first, when visitors post a comment, the comment usually is inserted in the front(you can also reset this of course),so they can see the comment directly. second, some people may not interested in reading comments, they read your post, and have an impulse to comment, so they dont have to go all the way down to the bottom, it is even worse when there is a long list of comments.
it is like Youtube.com. very clear and structured comment section.
In all, I would say it doesnt make any difference on performance and functionally, it really depends on how your think it is and what your website content is.