Is it ethical to withdraw a paper after reviews? I have a submitted a paper to a IEEE letter. Unfortunately, at a later stage I noticed that letter is still in process to have an impact factor. 
I already received reviews from reviewers and the journal is willing to 
accept the paper after minor revision. 
Is it ethical to withdraw the paper at this stage?
There is nothing to do with ethics as question asks. This is simple professionalism. The journal has devoted enough time for reviewing the manuscript. The reviewers have put in their efforts.

If you are really fixed about not go ahead with submission of revised manuscript, then do the following:


Don't submit the revised manuscript to the journal.
If the journal editor asks about the revision, then say that you are extending the article in various ways and would not submit the manuscript. But, thank them for their efforts.


But, I would suggest that you had made a mistake not looking at the journal statistics, you should go ahead with this journal with the publication.