I have an '87 and run a RB header and I had a local muffler shop make a header back, single exhaust that exits on the passenger side rear (normal location). It doesn't save a ton of weight but you get rid of one muffler, the cat and a little pipe. You could probably build it easily yourself with of the shelf muffler parts. I ended up having to replace my muffler after many years and got a new one from NAPA. I don't know what dB's it runs but I never have a problem passing, even at Lime Rock.
Brett_Murphy wrote: Rotaries don't like catalytic converters. Well, let me clarify. Cats rob power from rotaries, and using the wrong cat can result in a melted/clogged cat that will rob power and kill your engine.
Well that settles my first idea. 
My second idea is to run two lightweight racing mufflers with a turn down that you wanted.
I put a resonator and a Anza duel tipped exhaust muffler thing on one of my rx7s. It was remarkably quiet.
About catalytic converters: it looks like I'm going to be building yet another FC RX-7. I've got a factory RX-8 catalytic converter I'm going to weld into it. I don't like running without a cat, but that one should flow more than well enough for an N/A RX-7.
Is there a challenge way to get rid of that incredibly heavy intake manifold? That should shave off some lbs.
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