I have OEM mufflers but I used a Starla headpipe and intermediate pipes....the intermediates were fine, but the headpipe required some adjusting, the clearance and angle were off and to get a good fit took some heat and bending...no problems since. I have heard that some owners have found the flange bolts at the manifold/headpipe connection would loosen, though I'm not sure if they were Starla headpipes.
Ricardo