There is very little comparison
The Datsun works head is a very cleverly (and skillfully) crafted Honda K series scale up, and Honda heads are renown for making very high BMEP figures due to excellent port and chamber design
The P90 (in practical applications) can flow enough air to make figures over and above 300HP (350 with bigger valves, offset guides huge ports, big compression, over 3 litres and a well matched cam) where as the Datsun works head will EASILY flow figures that would dwarf that plus it has a very stable twin cam/lifter layout that will rev comfortably to 9-10K (HP = torque and rpms right)
The downside is that its still a lot of money, £10K will buy you the head and induction system but its not going to bolt onto a cooking or even mildly built engine and make big numbers, the bottom end would need decent crank, rods, pistons etc to get the rpms and compression up plus it needs a custom exhaust manifold etc not for the faint hearted
Budget circa £15K (ex labour) when its imported a decent L series short engine is built and set up.
But in the right circumstances on say a 3.2 or a 3.3 litre 400 naturally aspired HP and a noise to wake the dead would be something to behold
Conversly a crate V8 LS3 and T56 could be had for circa £7k and would make enough torque to put that new iceberg back in place.