the ITAC assumes the car makes the HP the OEM said it does, and generally assume a 25% gain over that number in IT trim. when a car came with PS ONLY, then the pump is supposed to be included in the test that derived the published HP numbers (SAE NET rules from some 30 years ago). cars without PS or with PS as an option likely DON'T have the pumps losses included in the published number.
the logic behind the rule has nothing to do with the Miata. but since you CAN replace a PS rack with a manual one, the Miata classification is correct on a single line. few other cars with any level of popularity have this option. when they do, their classification also allows them to change to the factory non-powered steering.
cars that come with PS are not at a disadvantage because the pump losses are included in the stock power numbers from the OEM, and the pump may be under driven in IT, in effect lessening a power parasite. the manual steering only (i.e. PS was not an option) cars don't have a PS pump to under drive and didn't have the associated losses reported in their OEM hp figures either (theoretically). so they are actually expected to make MORE power gain from the motor than the PS cars, if you think about it.
in either case, unless you upset an IT championship battle or piss someone off, running a de-powered steering but otherwise complaint miata in IT is not going to get you protested 99% of the time.
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