Just replacing the stock cats?
 

How much of the exhaust gains comes from removing / replacing the cats? From what I read they are the largest restriction for the whole exhaust system seems to be the front and secondary cat. If you just replaced these, would that be more cost effective?

No

The cats result in a minimal horsepower loss. A lot of the restriction is in the factory header / overpipe / front pipe on this car. After replacing those 3 components, there have been significant gains both on the dyno and the butt dyno.

I would plan on keeping one cat unless you enjoy your car sounding like crap.

I would only replace cats with high flow cats, I'd never run catless. Good info though.

I replaced the secondary cat with a high flow and have a resonated axle back muffler delete in 2.5" pipe. Definitely mid and top end pulls harder

I wanted to revisit this concept as I thought I read somewhere that someone replaced the stock cat with a high flow version and was able generate most of the gains of a aftermarket header (i.e. get rid of the torque dip).

I believe the stock header design isn't bad but the cat is a major restriction. Can someone help me find information on this?