The MonsterFlow probably offers the most performance for an NB:
http://www.good-win-racing.com/goodwinracing_003.htm
They claim 10 hp. Don't bother unless you get the CoolBox with it. Not
CARB approved in California, and foam filters don't filter as well as
OEM paper.

Or, if you don't have cruise control, the Randall duct is much cheaper
and delivers cool air while retaining the superior stock filter (5 hp
claimed):
http://www.flyinmiata.com/index.php?action=product&itemid=85465

In either case, don't expect more than a just-noticeable difference--on
a stock 1.8 Miata, the intake is not a source of restriction, though
cooler air is always good. Avoid K&N and other intakes that don't draw
directly from an outside air source--all you'll get is extra dirt in the
engine, and probably a power loss. The K&N advertisements are...let's
say highly imaginative.

Your performance dollars will work harder in the suspension than under
the hood. Alignment, tires, shocks, and swaybars will make real
improvements in your car's capabilities. A driver's school will make
real improvements in yours.

I've had the Monsterflow on a 2000.  It sounded great at WOT but the
performance gain was hardly noticable.

3 years ago, I did some testing of underhood temperatures.  As I recall, the
temp at the Monsterflow intake quickly went up by 30-50 degrees over ambient
when stopped.  Within a half mile after a stop light, though, it went back
down to ambient plus 10 degrees, the normal temp while driving.  From this,
I suspect that ducting cold air into the intake would do more than any air
filter.

I'm working with my third Miata.  I don't push the handling anywhere near
the limits car but I would like a little more zip around town.  The single
most noticable improvement I have found was advancing the timing (it can be
done with an M2, BTW).  Oh, I've made exahust improvements and tried cold
air intakes on both an M1 and M2,  and I found that I spent lots of bucks
and got little improvement for my money.  I've decided to either leave my
new Miata stock or follow Leon's example and install a supercharger -
anything else seems like a waste of money.

Randy