After a bit of digging, I found that the SiFive P670 has performance equivalent to the Galaxy S21, or the desktop Ryzen 7 2700, which is not too bad and definitely usable in a smartphone/laptop form, so competitive with the 2021 era designs. Definitely usable. It's not clear what's the power level is.
The P670 is a core, not a chip, so you can't really get to power numbers (or indeed, raw performance as opposed to performance / watt or performance / GHz) without actually making a practical chip out of it in some given process node. You're better off comparing it to a core, such as the ARM Cortex series, rather than the S21.
SiFive claims a SPECint2006 score of > 12/GHz, meaning that it'll get a performance of about 24 at 2 GHz or ~31 at 2.6 GHz, making it on par with an A76 in terms of raw performance.
