Spark plug indexing is common in many race applications. But it is one of those "extreme" things. The HP gain is only maybe a horse or 2, But when you are trying to get every last bit of "legal" power, you would use them. What they do is align the the sparkplug electrode in a position that will allow proper flame travel in the combustion chamber. And the different thickness washers are so you can get ALL the cylinders in the same position. To get optimum power and balance. Do it if you want, Won't hurt anything, but chances are you won't feal it.