Someone mentioned having a 2000 with a dual exit manifold with no precats? I have never seen this, I thought all later model XJ's with dual exit manifolds had the precats.
Alot of people mention borla and other headers, why go from a dual exit manifold to a single exit header, you probably wouldn't get any gains, maybe even losses. Not to mention you would have to weld on bungholios for the O2's. PITA
If you have a 2001 with the precats, maybe find a dual exit 2000 from the junk yard without the precats, if they exist... this would be a cheap way for a little extra gains on the top side of your exhaust but the 2001 manifold is pretty freeflowing as it is, compared to the earlier manifolds.
Other than that Clifford offers dual exit headers. Personally I ordered a kolak exhaust (carsound cat, flowmaster muff, mandrel bent tail pipe, and a bling bling tip!), I'll post my experiences after it is installed.