Regardless of what single high flow cat you use (or the Magnaflow you've got now), even if you don't have an MIL now, you will in 2000 to 2500 miles. Just my experience as the tiny cats cannot handle the heat and literally wear out and become inefficient. I look at aftermarket "high flow" cats as a wear item that just does not last as long as the stock dual setup. If they really do last like the originals, have the supplier put it in a written guarantee......I'm guessing nobody will. Hard to make a grapefruit sized cat last long with 600 hp blasting through it.

In my experience the side sills will still get hot even with high flow cats if you're on the gas under load for a long time. An example would be climbing long gradual mountain grades at 80-ish mph for 15 or more minutes. I've never felt my side sills that hot, even after track sessions.

Good luck with your project, I'm sure you'll find out where the lost hp is.