I'm in a similar boat. My XI has 140k miles. Due to a poorly operating CCV and aged Vanos seals my cats died prematurely last year and I am getting the age old P0420 / P0430 codes. I've since fixed and tuned up everything on this car but still get the codes.
With the age of the car and the mileage, I can't justify dropping $500 or even $1000 for new cats when mine are working, just not up to the DME's standards.
I've ordered a few components off Radio Shack's web site with the intention of building an O2 Sim. Hope it works. (All web sales of O2 sims seems to have dried up, so I am forced to build my own)