One objective of proper tuning is to insure the air:fuel mixture ratio that the carburetor delivers is correct.. another is to select the proper heat-range spark plug .. another is to set ignition timing for optimum performance.
All of these things leave tell-tale indications on the spark plug. Accurate spark plug "reading" can let you know what's happening inside the engine. You basically get a brand new plug, run it, remove it and examine it. The plug condition is like a photo snap shot of engine conditions at some speed.
You mentioned drilling holes in the air box which would allow more air into the carburetor. More air mixed with some set amount of fuel results in a "leaner" mixture. (also, more fuel mixed with some set amount of air would be a "richer" mix).
If a plug chop shows that the mixture is too rich, it might be proper to increase the amount of intake air (or to somehow provide more fuel.. same difference) thus leaning the mixture to the proper ratio.
Since engine damage can result from running too "lean" at top speed / top RPM, plug chops are most often used to determine proper fuel mix ratio at top speed, wide open throttle.
Get a brand new plug. Find a long flat road about half a mile or more long.
Thoroughly warm up the engine.
Run wide open throttle , top speed for about half a mile.
Kill (chop) the engine with the kill switch or ignition switch and coast to a stop. Do not let the engine idle.
Remove the new plug, install the old plug and ride home.
Examine the new test plug under good lighting and magnification .. look for signs of overheating, lean or rich air:fuel mixture, etc ..
If the new plug comes out looking just like it did when it came out of the box, the engine is in good tune and the proper heat range plug was selected.
Black soot on the new plug means the mixture is rich.
Signs of overheating (melted electrode edges or grey, pitted porcelain insulator etc.) could mean the mix is lean.. Or it could mean other things, like too-high compression ratio, too-advanced ignition timing, and other heat related engine problems.