K - I dont post here much - but wanted to try and help. Intakes are like ice cream - lots of flavors and they all are cold - meaning they produce about the same gains.
There is some good data out there for Miata intakes. Having worked in the Miata aftermarket industry and having sold a thousand of these things, the best bang for the buck is the
Racing Beat - Standard Intake.
The newer
CF Intake is quite nice.
The one that sold the best for us (Rspeed.net) was our
Monsterflow Unit. Great sound and easy design.
One of my favorites is the K&N short track intake - seen below:
Don't forget to keep checking
Miata.net 4 Sale Section.
I may even have some old Monsterflow parts around (now working somewhere else) - so building a good intake is easy - you just need to have a newer or decent filter that is clean and in good shape. Go with paper filter if possible as well.