I don't believe the plugs were the problem, if thats what's being told I would want reassurance that all plugs were removed from the shelf (assuming they were supplied by Race-Tech in the first place). It's not uncommon that an electrode from a spark plug falls off into the cylinder, however they don't "just fail", its usually down to an EGT issue, however the plugs would need to have their condition inspected. What grade plugs were used? Regarding which plugs to choose is quite irrelevant as it depends on the engines charcteristics, you also have to remember that a sacrifice has to be made, running a colder grade plug is fine for high speed running but may foul at lower speeds and vice versa. Another tip is that plugs from HKS, Greddy/Trust etc are all just re-branded NGK, Denso components.