Found instructions in the Power2Max FAQ on how to determine if the unit calibration is correct:

"To minimize the amount of time you are, or might be, without a power meter we’ve developed a test that you can do to determine if your power meter’s power values are outside where we would expect them to be.

Here’s what you’ll need:

The weight of you and your bike (in kg)
A hill at least 1.5km long and 5-6% in grade
The actual length of the hill
The height difference (altitude gain)
And total time

I dug through the recent rides and found an effort of 1.9 miles @ 5% average, and plugged in the numbers. The PM on the bike gave me a 249W average, and the calculator linked by P2M (all in German, naturally) gave me 255W, which is within 2%. So I guess the Stages was just reading quite a bit high for... the whole time I had it.