A healthy rotary will still end up killing the OEM cat. The exhaust is still really hot, and there is still a lot of excess hydrocarbons in the stream. The OEM cat can deal with it for a while, but not forever. I'd guess at an average of 80,000 mile lifespan for an RX-8 cat as long as there are no ignition problems or floods. Either one of those will quickly chop life off.

Thanks for the replies.