There's nothing wrong with a white insulator..
The indications of a dangerously too-lean mixture is that a top-speed plug chop on a brand new spark plug shows efinite signs of overheating.. The porcelain insulator will be pitted, grey or spotted with bits of aluminum.. or it's electrodes will show melting at their sharp edges under magnification.. And look down deep inside the insulator. If, after the plug chop, you see any black soot deep down in the bottom it's from unburned fuel, and the mixture is not too lean.
First check and see if the combustion chamber has a lot of carbon collected inside. This carbon build-up reduces the combustion chamber volume and it raises your compression ratio. A compression ratio that's high causes the fuel mix to burn hotter and faster.. if there's a thick layer of carbon, carefully chip and clean it away..
Even a small piece of carbon in the chamber that remains red hot can cause preignition .. no spark is needed since it's red hot and ignites the fuel mixture.
Another thing that happens is people use the wrong spark plugs.. if it's too long the edge of a thread of the spark plug protrudes into the combustion chamber and gets red hot.
Or, any sharp bit of protruding metal in the chamber can get red hot and stay hot.. and it ignites the mix before the spark does.
Assuming the combustion chamber is clean, the cures for pre-ignition vary .. you could drown the engine in fuel with a bigger main jet but that will kill power output. .. You could cut back on oil a bit to richen the mixture..
Or you could retard the ignition timing a bit .. OR you could use a higher octane fuel. ..Or you could lower the compression ratio with a thicker head gasket.
I know you've already tried some of these solutions.. and it's very hard to believe that an unmodified, completely stock, brand new bike would have a preignition problem at +25mph.
If there's more to the story of the history of the bike, say so.. like if someone shaved the head.. or "forgot" to reinstall a cylinder base gasket .. or messed with something else..