SparkPlugs
A spark plug (also, very rarely nowadays, in British English: a sparking plug) is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed aerosol gasoline by means of an electric spark.
Info from the GS500 Owner's Manual:
DPR7EA-9 If the standard plug is apt to get wet, replace with this plug.
DPR8EA-9 Standard
DPR9EA-9 If the standard plug is apt to overheat, replace with this plug.
Iridium Spark Plugs
Iridium is a chemical element in the periodic table that has the symbol Ir and atomic number 77. A dense, very hard, brittle, silvery-white transition metal of the platinum family, iridium is used in high strength alloys that can withstand high temperatures and occurs in natural alloys with platinum or osmium. Iridium is notable for being the most corrosion resistant element known and for its significance in the determination of the probable cause of the demise, by a meteorite strike, of the dinosaurs. It is used in high temperature apparatus, electrical contacts, and as a hardening agent for platinum.
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As you can see the Iridium plugs have a much smaller centre (center for you yanks :P) electrode. The smaller the electrodes surface area, the easier it is for the spark to jump the gap. This reduces the chance of miss firing especially with lean fuel to air ratios.
We all know the GS typically runs very lean, and that motorcycle engines don't have the strongest electrical systems, so any help you can give the system is a bonus.
The small 0.6mm electrode NGK used is made from a very special metal known as Iridium. This is because with softer, more commonly used materials like Nickel, Gold, Silver and Platinum would wear away too quickly at this small size. The standard NGK plugs centre electrode for comparison is made of Nickel and is a little bigger than 2.0 mm.
The side electrode also has its side ground away. The minimises the amount of metal over hanging the spark area, allowing the flame of ignition to spread faster and more evenly.
A more efficient spark plug in theory results in more power, better fuel economy and reduced emissions.
iridium plugs come pre-gapped. There is no need to adjust them at all.
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Now as for NOTICEABLE performance increase.... from the seat of my pants I have noticed nothing. But then again a 1-2% increase (about the maximum you could ever expect from spark plugs) would not be very easy to detect by the seat of your pants.
I did this more for the peace of mind that anything that helps to reduce misfires is a good thing!
Reducing misfires equals better acceleration and power delivery.
A side benefit is you can keep them in motorbikes for up to 32,000km / 20,000 miles! (96,000km / 60,000 miles in normally aspirated cars!)
The only draw back of these spark plugs is the price as they were 3-5 times dearer than the stock plugs.
Side Gapping
Taking a standard spark plugs and grinding away some of the the top electrode, so that more of the spark is exposed to the un-burnt fuel.
This will improve engine power but dramatically reduces spark plug life, and they will need to be changed more often.
Great for track days, probably not the best for everyday street use.
Electrode Positioning
Pointing the top electrode so that it is pointing towards the centre of the combustion chamber. This exposes more of the spark kernel to the fuel/air mixture allowing for better combustion and more power through better efficiency. This will also improve fuel consumption.
Mark the spark plug with a marking pen so you know where the open end of the spark plug electrode is, and screw it into the engine block as tight as possible but allowing the electrode to be in the right position.
NGK
NGK product numbers for the GS500 Standard: NGK DPR8EA-9 (gap 0.8-0.9mm) reference Iridium: NGK DPR8EIX-9 (Pre gapped from factory)
Denso
Iridium: IX24B (Pre gapped from factory)