http://www.tomorrowstechnician.com/Content/Site301/SmartProducts/RotaryEnginesTe_00000001985.pdf
Just stumbled upon this document describing Renesis changes; one of the things mentioned is the change in spark plugs. Looks like Mazda was concerned about plug tip temps, even on street engines.

Charlie


Micro-electrode spark plugs
The last technology employed in aid of fuel economy for
the RENESIS engine is the micro-electrode spark plug.
This spark plug uses a small side electrode and thick
gauge central electrode with an extremely fine tip that
promotes stable ignition of lean air-fuel mixtures. Also,
by maintaining a lower temperature for side and central
electrodes, the plug achieves high heat-resistance. The
tip of the central electrode, which was previously platinum,
is now made of longer-lasting iridium.