NC used the MZR engine, which was also the mass-produced Mazda3 engine at the time. The NC2 added forged rods and crank, but the Skyactiv engine has those in the Mazda3 already.
The ND version of the Skyactiv has been tuned for 91 octane. The torque peak is marginally higher, and the throttle response is significantly improved, especially in the low range. There was an interview somewhere with Dave Coleman where he explained that part of the reason for DBW throttle delay is to control detonation from snapping the throttle open too quickly.
EDIT: There's a really good and thorough review of the Skyactiv tech here:
http://www.motoiq.com/MagazineArticl...echnology.aspx