+1 for cycleman's explanation and results.
Others have had similar results even documenting a 2hp gain on Yamaha xs650 and 750s on dynojet testing (see http://www.xs650direct.com/products-27.html
As the xs engine has a similar combustion camber shape as the airhead I thought it would be worth a try. Yes the plug is expensive but if it extends the service life and if it improves the engine performance even marginally it would be worth the extra cost. The reduced voltage required to fire a "pin" type electrode should provide reduced fouling and improved starting (or so the theory goes).
While cycleman may well be right about not worrying about the plugs built in 5k resistance, I tend to like to duplicate the original ignition specs as closely as possible so as not to stress the coils.
Just my thoughts and I'm no expert.