Just reporting back... since finding this thread, I cleaned mine.. 2004 WRX with 98k and I'm assuming it's never been touched. It was dirty, but not as dirty as I would expect.
Didn't do a damn thing for mileage or for my high AF learning values. Turns out, it was a leaky BPV gasket causing that.
As for cleaning the MAF, I'd only recommend it if you have a MAF related problem.... or 'maybe' every 100k. Butt dyno didn't change, even after resetting the ECU and letting it learn.