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You will notice more coma in an f5 or lower ratio Newtonian than in an f6 system although it is still present in all. Most users can live with the coma at f6 but most users use a Paracorr at f4.5 to f5. f5 is the fence where some users fall one way and others the other with regards to whether the coma is objectionable or not. An f5 system too requires better corrected eyepieces than an f6 system although the difference is more noticeable from say f5 to f8.
With regards to better corrected eyepieces even Tele Vue (highly corrected and tested to f4) will show coma as it is a function of the primary mirror not the eyepiece. What they wont show is astigmatism which poorly corrected eyepieces are masters of. In fact seeing coma alone with no astigmatism is almost pleasing (until you put in a Paracorr and stars are pinpoint to the edge!).
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