I recommend the Jackson Racing Cold Air Intake for the miata, for any year. Purchase price is a bit steep at about $400, but the horsepower and torque gains are every bit worthwhile. The majic of this system is the incredibly shortened intake plumbing and the well-designed intake box which mounts just aft of the radiator and in front of the engine. This allows the system to draw air at near ambient temperatures as it sneaks under the hood from between the front bumper cover and the hood. You will note that I don't have hard dyno numbers for any of these modifications, but I can give close estimates to expected power gains.
The aforementioned cat-back system should yield around 3 horses, and combined with the JR CAI you should see an increase of 7 total horsepower at the WHEELS (not at the crank) over stock. You do the math, but even the most competitive alternative intake systems don't come close to these numbers.
2. Racing Beat's Polar Power Cold Air Induction: ~$275.00?
*Draws intake air from same area as the JR CAI, but retains the stock airbox and plumbing. No idea of the horsepower gains from this system, but it can't be much if any more than the K&N unit.
3. Racing Beat's cone-type filter with aluminum elbow: ~$200.00
*Same idea as the K&N unit, wiht the added advantage of drawing intake air from a cooler portion of the engine compartment. Still, it's just not the JR CAI.