Moakster,
What year Road King did the pipes come off of? There were certain later model years that had catalytic converters in them. I think they started with the 2006 and forward but it may have been earlier.
I have the slash cuts on my '03 and other than the sound my bike is just as it was. I punched the baffles right out of them so I could seen straight through muffler. I think I get a bit better mileage than I used to but no vibration or lack of acceleration.
Cheers,
John