Yes the Nelson and Turbo plugs are different and will need a custom made head, but the head has a flat disc that could fairly easy be copied (some people seem quite skilled on this forum...) to take a different plug and thereby avoid the influence of the volume in the plug-threads on the combustion?

The Merlin plugs I assumed were standard threads?
They have plugs labeled 1/2a that they claim will add 1000 rpm...