What's the issue with this plug at high power levels? is it reliabily (ie melting electrodes) or missfire problems?

Rick.

 

its more heat range and melting point

 

As above, if heat range is wrong for the engine, then the tips melt, which could = disaster.

They're a good plug when used in the right application.

 

I think Stage three is the most they can do, anymore and you risk the tip melting off and scrapping your engine
I would avoid and try the Ford apr 701's



Dave.

PS These are RS turbo plugs and cost less than a tenner, as recommended by Mike Rainbum.