OK, after talking with several reputable shops and way too much time searching on the internet, here is the conclusion that I have come to: the problem is in fact with the aftermarket, hi-flow cat, but not because its bad. The consensus is that the Magnaflow cat is not getting hot enough and is building up crud on the honeycomb stuff. Recall that I am a self-admitted feather foot. Apparently, Magnaflow now makes a hi-flow, "California" OBDII cat that is less restrictive than a stock cat but gets hot enough to always do its job with respect to emissions regulations. I dumped a can of BG 44K in a new tank of fuel and went on a 400+ mile road trip through the Colorado mountains. I ran her harder than usual, up and over Eisenhower and Vail passes a couple of times. Finished the trip at 22 mpg and no CEL has come back on in the past 500 miles. I'll update this thread if anything changes, but so far, this is the working hypothesis. For what its worth, this issue of hi-flow cats not getting hot enough seems to be really common if you search the discussion boards for the Honda street racers.

Hope all this investigation helps someone in the future!