[Ant]

My engine has not sure of # has heat grade 6, most road cars run either 5 or 6, and it appears 7's are too cold for your use. I have run this engine on the dyno with the 6's as the engine builder supplied with 675bhp.
I have run 10's in racing engines with never a problem with fast road use, a good ignition makes a difference like MSD with a good multiple spark which helps to avoid plug fouling. I reckon most people fit a colder plug for peace of mind as they help to avoid detonation etc or a bit of insurance when racing!