The stock part is apparently very restrictive and I'd like a freer flowing setup. There is one available for the Roadster that is also a cat in the muffler type.
Yeah, examining the stock part you can see how the flow is split between the two pipes causing two 180 degree turns before exiting one side. That said, I'd think the cats create more restriction than the box. But, if you want to keep the cats for their emissions properties I can see how a crossover might give a marginal gain.
I bet if you pulled the stock part off and took it to a muffler shop they could fashion a straight through replacement. Right sized pipe, a few bends and you'd be in business.