So after some more research, it seems that the metallic substrate catalytic converters, like the GESI are more efficient or cleaner than the ceramic substrate ones
on a per volume basis. That means that running what amounts to a 4" x 4" (body only) cylinder of metallic substrate catalytic converter (specifically, the GESI 50025) would likely not filter as well as something like a 4.25" x 9" (body only) cylinder of ceramic substrate, like the magnaflow OEM-grade cats.
This post if from an RX-7 forum, but details quite a bit from some source materials about the comparisons of the two types. This is where the majority of my useful info came from.
A major fault in my conclusions may be that GESI has overcome or outperformed what's used for examples in the source, which is about 10 years old. There's not really an unbiased way to find this out without spending a LOT of money doing firsthand testing. That's not in the cards for me, personally.
Assuming the data is still valid, what does it mean? I'm not 100% on the real world results, but it would seem to mean that you'll likely see much less restriction from these GESI cats, but also less total filtration. I'd expect that the reduction of exhaust smell would be lower as well. For me, the biggest factor in not running catless is the smell, followed by the emissions reductions. Given this, I don't think these GESI's are going to be a good fit for me.
One big advantage to the metallic substrate is that they'll withstand much higher temps, and are good candidates for use really close to the engine. An ideal situation would be running the GESI as the primary cat, and then a larger ceramic catalyst as the secondary ones, but that runs into an arena that would be cost prohibitive. You'd be looking at a fully custom setup with $1000 in catalytic converters alone.
On the flipside of this, the ceramic cats are going to likely be more restrictive and better filtering of exhaust pollutants. Overall, though, if I'm eliminating the primary cats, and eliminating the crushed, double walled factory y-pipe, I suspect the net effect would still be freer flow overall.
I'm still circling back around to the idea that these oem-grade magnaflow y-pipes are probably going to end up being the most effective use of my time and money, given that any other option has the added risks and expenses of fabrication. The magnaflows are 100% bolt-on, mandrel bent, 409 stainless, true y-fitting, and with what is likely 400 CPI ceramic substrate.
I'm going to keep mulling things over and keep researching.
