I think the br8es plugs would be too "hot" of a plug. The 8's are usually only used in special applications. The engine may not be broke in yet. New engines are still "tight" and don't make quite as much power as a "broke in" engine.
Run the 9's. The heat range number represents the ability of the plug to get rid of heat. Going to a hotter plug doesn't increase power, but it may create a hot spot and cause problems such as pre-ignition. You should always use the plug the engine was designed to run.
Polaris actually specifies a Champion RN2C plug and not the NGK. They are considered comparable, though.
2007 Dragon RMK 700 155", 2008 RMK 600 155", 95 Prowler 550 2-up, Really OLD AC Cheetah