The article below is taken from the NGK spark Plugs site, they think it has minimal advantages.
"Gapping and Indexing
Gapping
Since the gap size has a direct effect on the spark plug's tip temperature and on the voltage necessary to ionize (light) the air-fuel mixture, careful attention is required. While it is a popular misconception that plugs are pre-gapped from the factory, the fact remains that the gap must be adjusted for the vehicle that the spark plug is intended for. Those with modified engines must remember that a modified engine with higher compression or forced induction will typically require a smaller gap setting (to ensure ignitability in these denser air-fuel mixtures). As a rule, the more power you are making, the smaller the gap you will need.
A spark plug's voltage requirement is directly proportionate to the gap size. The larger the gap, the more voltage is needed to bridge the gap. Most experienced tuners know that opening gaps up to present a larger spark to the air-fuel mixture maximizes burn efficiency. It is for this reason that most racers add high-power ignition systems. The added power allows them to open the gap, yet still provide a strong spark.
With this in mind, many think the larger the gap, the better. In fact, some aftermarket ignition systems boast that their systems can tolerate gaps that are extreme. Be wary of such claims. In most cases, the largest gap you can run may still be smaller than you think.
Indexing
This is for racers only! Indexing refers to a process whereby auxiliary washers of varying thickness are placed under the spark plug's shoulder so that when the spark plug is tightened, the gap will point in the desired direction.
However, without running an engine on a dyno, it is impossible to gauge which type of indexing works best in your engine. While most engines like the spark plug's gap open to the intake valve, there are still other combinations that make more power with the gap pointed toward the exhaust valve.
In any case, engines with indexed spark plugs will typically make only a few more horsepower, typically less than 1% of total engine output. For a 500 hp engine, you'd be lucky to get 5 hp. While there are exceptions, the bottom line is that without a dyno, gauging success will be difficult"
