Unfortunately the engineering reality is that speed and endurance (particularly at low altitude) pull in opposite directions. I believe that at best the P-8A equals the TOS-at-radius of the P-3 despite being much bigger.
One problem (if the USN's own numbers are correct) is that the P-8A can barely take off with full tanks, even with no weapons on board, and appears to have a fuel fraction lower than that of the P-3C (that's a bit of a squishy number because the P-3C would have gained weight during its career). That will tend to mean less range and endurance, particularly at low altitude.