Decisions made! Parts ordered!
I'm going to go ahead and cut out the 1.5" restriction at the end of the header and run 1.75" all the way back.
I got a high-flow 1.75" cat, 1.75" muffler and 1.75" resonator. The mandrel J bends that I got to put it all together are actually 1.875", and I bought a cheap pipe expander. I figure with extremely little expansion, I can make the pipes slip fit and maybe be easier to fit up.
Did a little more research on cats. I'm sure I read all this before, but obviously, I needed the refresher. There are two kinds of cat substrate. There's the typical OE style ceramic material, and there's the newer metallic material. Both function the same as far as converting gases, but the metallic substrate is MUCH thinner, and thus offers considerably less restriction. So, the cat that I bought isn't actually labeled as a "high flow", but it is a Magnaflow spun cat with metal guts. And being 1.75" (when I only need 1.625" max), it should flow plenty for my needs. It was pricier than the 2" cat that I was looking at before, but the 2" was ceramic. What's another $40?
Bottom line, I'm in for $219. That includes the $20 pipe expander and almost everything required. I'll need to find a flange gasket, some material for hangers, and schedule some time at Joe's Garage to put it all together.
Parts should be here by late next week. I might have time to work on it sometime soon if I can coordinate with my buddy, Joe.
All else being equal, if I just replaced the OE cat with a good high-flow cat, I'd probably gain about 1hp across the board, and maybe 2 at the top end.
Getting rid of the 1.5" bottleneck throughout the entire system should free up another couple hp.
Net should be pretty close to what SpeedCircuit's dyno shows with the header and no cat.
I'd be pretty happy to just keep the gains that I already have and add a little more on the high end.
And since I'm using a non-straight through main muffler, and a resonator that is a few inches longer than stock, I don't expect it will be much louder. Larger pipe will let more sound through, but I don't think it will be a lot.