Take a look at
http://mike.focke.googlepages.com/checkenginelightcausedbymassairflowsensor and Boxster TSB 2445 (Air Flow Sensor) dated Apr 6, 2001 (if you can get access to it). The TSB specifically says the earlier E-gas MAF (996 606 124 00) needs to have the DME reprogrammed when replacing the 996 606 124 00 MAF with the newer 986 606 125 00 one. I also found out yesterday it is recommended you disconnect your battery for 5+ minutes to clear out all stored values so your DME can relearn the MAF programming
Supposedly DME programming at a dealer with a PST2 or PIWIS is about 1/2 hour labor charge