Well, I finished the 3" muffler section to mate to BEGi's new turbo kit for the NC miata.  It has a 3" inlet pipe which I ovalized to match the two inlets into the 2.5" core mufflers.   Begi made 249 hp with a 2.5" system.  I'm excited to see how much improvement is available with the full 3" midpipe and muffler.   The difference between 2.5" and 3" may not sound like much but it is a 44% increase in cross sectional area.