With headers and exhaust ( and tune ) the new Miata gains +50 hp and +50tq thanks to its factory 2.0L engine and 13.0 compression ratio. We are talking 193 hp and 190 tq to the wheels ( 139 whp is stock ). I think it's impressive.
Source:
Good-Win Racing Longtube RACE Header OVER 40HP! - MX-5 Miata Forum