Yup, I know the C320C31 - used them at GE back in 2008: as you say, quite fast, and cheap.
And yes, 2^n byte length circular buffers: one of the most essentially elegant constructs in the whole of programming, eh?!
2^n is the buffer size, and (2^n) - 1 is the rollover mask: it doesn't get any easier than that :-)
Of course that's a feature I'm using in my digital AGC project too...