For sigmastudio, though, it all comes down to instructions per cycle. You have fixed amounts of instructions that you can feed, mostly dictated by the size of memory, but also limited to the IPC/IPS of the ADAU1701 chip itself. At a lower sampling rate, it actually uses fewer instructions available for the chip in sigmastudio. And every saving of instructions there means more coefficients may be possible.
I also glossed over the fact that implementation of IIR filters can shift time alignment done through the sine-sweep method of impulse response. That is also why I recommend the time alignment between drivers before PEQs are applied. I left links to an article on the topic in a footnote, but that is a little easter egg if you care to read them.
But for the -40, good to know that was for rephase, not the signal detect / mute function.