I've run the Piedra quite a few times at varying water levels (400 to 2500 CFS on the Arboles Gauge) in rafts and kayaks, commercially and privately.
There are many names for the two rapids in the upper box or "2nd box" that I have heard. Generally the upper one is called -Lone Pine- and the lower "s-turn" rapid, -Limestone-.
Lone Pine is a relatively techy boulder garden that I believe to be between class III+ and solid, yet not strong, class IV at most water levels for rafts and kayaks. However, it is a very busy rapid and probably seems more difficult to people who don’t frequent the rapid. There is a pretty III+ to IV- line far river-right at lower water.
Limestone's character changes more or has a wider range than Lone Pine. At lower water I would call it class IV- because of its close-quarters constriction and obstacles. At higher water the entrance hole becomes sizeable and requires some momentum to punch. The S move must be executed in less time. Also, there are a couple big holes after the S turn, especially the one on the very bottom river-left (perhaps the one that chundered you).
At higher water I would venture to say Limestone is at least solid IV, if not IV+.
It’s all interpretation though…