Found this page that says that a 'thermal failure' will set a P0420 code. Since you've only seen the high temps in short runs I doubt you've set a code.
http://www.catcoglobal.com/assets/fi...ld%20final.pdf
Also found this:
Typically, converter temperatures will not exceed 1200º F on a properly running engine. Periodic operation above 1600º F can negatively affect the precious metals coating on the substrate, reducing its efficiency. Excessive temperatures can reduce the converter's durability, or - if high enough - destroy the converter's matting or substrate.
on this page:
http://www.walkerexhaust.com/support...er-temperature
Sounds like your temps are at a problematic threshold. I would guess that something like a track day might cook your cats. Occasional spikes that high are probably safe, but I'd check logs running at less than max boost to see if you're in the safe area (didn't go back to see if you'd done that).
Also, if nobody else bites the bullet I might order the Simon 2 cable. What's another $45 at this point...