When I researched Catalytic converters a while back it looked like there were three or four high flow cats that might be useful.
Random Technologies
Car Sounds
CatCo
Marmont
CatCo sells lots of different cats but only the 2600 series could be considered high flow. Since CatCo isn't on the web I called them and they claim higher numbers than Random for the 3" Cat.
Hahn used to sell Random but switched to Marmont. I never sound out exactly which model but Bill Hahn claimed the Marmont flowed just as well and the Random but was much less expensive. That thread in the archive here somewhere.
Car Sounds seems to be popular in other auto circles and has some interesting features
Dynomax didn't seem to make anything in 3" that would fit in the same space as the stock cat.
I haven't bought a new cat yet but for the money I figured the CatCo 2606W (2.5") or 2607W (3") seem the best choice. The W suffix means without an air injection pipe.
Steve