I did a sort of comparison test on my bike when I replaced the NGK BP6ES conventional plugs with Denso IW20 5306 Iridium. I paid $34.41 for six, including $6.99 shipping, from RockAuto.
"0.4mm Iridium tipped laser welded fine wire center electrode reduces required voltage and improves firing performance.
Tapered cut U-Groove ground electrode increases ignitability and decreases quenching"
www.densoproducts.com/product.aspx?zpid=11175
1) The plugs in the engine were over ten years old, but showed no signs of wear. I cleaned them with carb cleaner, then heated them with a propane torch until they glowed to burn off any residue.
2) The Denso IW20 part #5306 plugs had the large connector, but the tip can be screwed off to reveal the threaded connector that these bikes require. HOWEVER, since these plugs are also used in automotive applications, some manufacturer cross-over tables list as equivalent to the BP5ES, plugs with the large connector that DON'T screw off. Be sure to check the details if you decide to use a different brand.
3) After the switch, the bike started up much more quickly than before.