Do you think hollowing out the VX cat would be bad for FE...the exhaust velocities would be greatly reduced b/c it's basically dumped into a large volume, so you'd loose the scavenging effects. wonder how much a custom small diameter equal-length header could help??
If you want a small diameter header, just use a stock exhaust manifold from a DX/LX/EX/SI Civic of the same generation that the car is going into. I do know that there are some differences between the various generations. For instance, the 1992-1995 EX/SI uses a 4-2-1 design, while all the others use a shorty 4-1. But you could experiment to see what works best. Regardless, ANY of these should be better than that stipid close-coupled design that the Honda engineers decided to stick the VX with.
I should also mention that the MAIN reason for hollowing out and still using the stock VX cat is for emissions testing. Here in California, they are going to look for a cat in the stock location. So it would be necessary to swap to this confiuration for testing.