(Hopefully this question is not too "soft": I asked it at math.stackexchange.com recently and was advised to ask it here instead).
I submitted a paper to quite a low-rated journal (impact factor less than 0.3) a while ago, partly because I didn't think that my results were particularly good, but also because at the time I was largely ignorant of the significance of journal rankings. After many months "with editor" it has now finally been sent to a referee, but I'm slightly regretting choosing this journal, as recent feedback has given me the impression that my paper is publishable in a much better journal, and I am at the point in my career (starting to look for my first post-doc position) where I really need some good additions to my publication record.
Now that someone is possibly taking the time to read it, would it be considered bad form to withdraw the paper (or wait until the review comes back, assuming it is favourable), and submit it elsewhere? And is it worth the risk? When considering job applications, how much emphasis do people actually put on where a person's papers have been published, as opposed to how many have been published?
Any advice much appreciated.