Originally Posted by Jordo!

Oh, there's a few...

1. The aptly nick-named "demon bolts", as another member noted.

2. Unforgiving rasp and rattle on decel (if unresonated TP's are used).

3. The potential for a CEL if the ECU detects too much similarity in voltage changes between primary and secondary O2's.

4. Passing emissions, if that's a thing in your state; (NB: #3 and #4 are both possible problems even with HFC's, and resonated HFC's are advised as well...)

5. Possible fuel odors (probably more for those running TP's untuned...)

6. A tune is not "required" but is highly advisable. Pulling the cats makes for significant differences in flow and also means less heat at the head, so you can dial in more timing -- the OEM cats are arguably the greatest bottleneck on the car, and swapping the cats out for either HFC's or TP's is a high bang-for-the-buck mod in terms of power gain.

Nothing insurmountable, but not quite as easy as just swapping out the cat back and calling it a day.