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.
This is mostly true but does not provide a complete picture. Yes, the ToS at radius of the P-8 is similar to P-3, and P-8 is bigger than P-3, but its bigger for a reason. The Navy wanted lots more "stuff" on station. If you look at Lockheed's proposal that lost against Boeing's, Lockheed's solution was not merely an upgraded P-3, but an entirely new aircraft significantly larger and MUCH more powerful than the P-3. Nimrod was bigger than Orion also, and for good reason. It was a superbly equipped and capable aircraft.
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).
USN specified additional tankage to the P-8 sufficient to reach MTOGW with a light weapons load. As for the "lower" fuel fraction, that was driven by the equipment USN added to the P-8. If the P-8's full mission suite had been added to a P-3, the P-3's Operating Empty Weight would by very high and its fuel fraction would have been laughably low. Basically you can't really compare a P-8 to a P-3 (just as you can't really compare a Nimrod to a P-3, or a P-3 to C-295) because they are very different aircraft with quite different mission suites.