Taken from a Mazda press release:
The 2006 Mazda RX-8 is available as either a 232-horsepower model fitted with a six-speed manual transmission, or the new 212-horsepower model fitted with a six-speed automatic with steering-wheel-mounted paddle shifters for a Formula 1-style driving experience.
Both horsepower figures are SAE J1349 Certified Power, which is a new method of rating vehicle power for 2006. The SAE J1349 is an automotive standard written by the Society of Automotive Engineers to provide manufacturers with a method of certifying the power of engines.
The press release can be found here:
http://rotarynews.com/node/view/757
So it would appear that it'd be a good bet to use the 232 horsepower figure for classification of the RX-8 starting with the 2006 model. Updating and backdating would allow 2004 MY RX-8s to use whatever it is the later models have to produce the J1349 certified horsepower.