Miata?

[Jocassee]

It'd be more effective if it worked.

One of the street accessory stores near me has one, not quite that high but pretty close, that works.

[dogmush]

Removing the cat (on a streetable performance car) is a holdover from a bygone decade, when the thing actually caused excessive back pressure. Modern OEM cats flow pretty good, and the aftermarket performance cats are going to flow pretty close to having none at all.
Now if you are running some highly competitive series with a dedicated car, like spec miata, where the extra 1hp is worth having, then by all means...
http://youtu.be/yubpZk9cIFo?t=46s

But for the vast majority of us, there is almost no performance difference, and a cat makes a big difference to the poisonous fumes expelled from the engine, even a high flow performance model.

I run a high flow cat on my aftermarket exhaust (everything from turbine outlet to muffler inlet was replaced with larger piping). The new system flows so well that the stock wastegate can't hold OEM pressure target (7.5psig), now it runs consistently to 11-12psig at full throttle. Fortunately I have injectors and ecu mods to keep the afrs in line. I could have gone to an even larger exhaust with cat, but that would require taking the turbo off to port the wastegate as the boost creeps to dangerous levels with that system. Point is... the cat is not a limitation on flow.

Ehhh. I get like 35 HP difference between my street midpipe (w/cats) and my off road pipe.  Same pip diameter.  Measured on a dyno.  But I'm prolly pushing more air then you.  I'm also running manifolds.  Longtubes would bring the two pipes closer together in performance.