I've searched, looks like the Magnaflow 3" Metal Core Spun Catalytic converter is pretty common on boosted setups. Like this: http://www.summitracing.com/parts/mpe-59929/overview/

I also found this Thunderbolt for much less: 3" Catalytic Converter High Flow Thunderbolt Obdi | eBay
But it looks much more restrictive and I'm guessing that's a ceramic core not a metallic core. I dunno which is more durable on a boosted miata?

I plan to leave it on the car all the time, so I need it to not be too restrictive and last a while.

 

Magnaflow 59959 on e-bay for less than $100. Same internals as the 29, but shorter, so easier to fit wherever you put it.

 

You can get the thunderbolt in bother ceramic and metallic core. I think metalic is more money, 300 cells per inch for the metalic. On cars that I've seen run both the magnaflow and the thrunderbolt the thunderbolts normally last 4-6 years as opposed to the less than one year the magnaflows were doing.