The P8 is not designed or meant to operate the way the P3s or Sunderlands have. Just as the P3B was a big step up and sea change in ops and methodologies from the Sunderlands so the P8 is from the P3K2. Low and slow may not be needed nor actually desirable anymore with SL-SAM capability being added to subs, so if you come in low and slow you may likely eat a SAM. I would suggest that people read this
The Arrival of a Maritime-Domain Awareness Strike Capability: The Impact of the P-8/Triton Dyad. The title says it all and it's no longer just about ASW and ASuW but about
Domain Awareness; how to access it, manage it and exploit it. The P1 does not have this capability to anywhere near the extent that the P8 does. It's the same concept that is used in the C5ISR aspects of the F35 which make that aircraft far more than just an ordinance truck that happens to have some LO capabilities. The C5ISR aspects far outweigh the F35s ability to drop bombs, fire missiles or shoot at the bad guys. That's what a lot of people don't understand. The same goes for the P8.
Then think about the cost effectiveness. Less platforms are needed to cover a given area and NZ has operated for decades with the absolute minimum number of platforms for the size its AO.