It has been many years since California has been offered a legal horsepower bump
for the Miata.  Legal meaning CARB-approved.  An owner doesn't have to worry
about bi-annual smog inspections, removing equipment, finding a certain type of
smog station, or [even] further polluting the environment.  Up until recently the only
legal upgrades currently being sold applied to first-generation [NA] Miatas.  Then
earlier this year Flyin Miata announced that they had [finally] received CARB
approval for their supercharger platform for the NC.  Now, however, we have been
surprised by the recent CARB approvals of two boost solutions for the ND Miata.  
Both cars passed cleanly on their first attempt.  Kits should be shipping soon, and
both vendors are taking orders.  Neither kit is cheap, and owners may want to also
consider further investment [uprated clutch, oil cooler, increased radiator capacity]
for cars that will see the track.  But... the options are finally on the table for those
that want more power.
Links to relevant threads on the Miata Forum

Edelbrock Supercharger
BBR Stage 1 Turbo