For the curious, here is the same plant now, 8 years later, back in the ground and twice as tall. Long story but I don't plan to dig it up again.
If you look at the constrictions in the stem, you can resolve individual seasons of growth. Looking that way you can see this past season (the plant's first full season in the ground again) it grew 3 times as much as it was doing on average in pots.