If the mod caused detonation one would only need to watch one of the many engine monitoring devices to see if timing pulls back.
Also (not saying this is a good idea either) but with a decent machinist one could leave a "sleave" to cover the thread end of the plug, the volume reduction would be less but then it would just blow the sleeve up before the threaded hole. (hollow plug)
Its his car and his money, if it works for him more power to him, would be interesting to see what happens in 100,000 miles (if anything)
I have a feeling this mod could cause more irritation than stock but likely little else.
Helicoils might happen though.
Good Luck
Ryan