Theories I have that need explanations about my new used exhaust- Have a random classic for your troubles if you read this

So Oppo, just yesterday I had my exhaust replaced from the downpipe back on my 04 WRX. I found a CL ad for an 06 STI turbo back exhaust for 250. My situation was I had a crack in my catalytic converter (after the midpipe) and on top of that a lot of New England rust keeping everything "nice" and "snug". So anyways, now that the new exhaust is on I've noticed that high rpm shifts between 2nd gear and 3rd are no longer as jerky. By that I mean, as soon as I put the clutch in to up shift my whole car would bog down noticeably (like someone pushing it backwards) until I got back on the gas hard in 3rd. More rambling and some theories ahead...

Now when I shift in high rpm's (when the turbo is spooled) there is not as much bog it seems like. It feels incredibly more smooth. It may be because I don't often really go nuts with my RPM's and shifting. I may be just feeling a placebo affect because I except things to be better. Yet this feels different. It feels clean and smooth like how I envision a newer car to act (i've never had a new car).

So my theories as to why all of a sudden everything feels better. These are probably wrong which is why I'm posting this ramble. I like learning about things that confuse me and I figure that's where you guys come in.

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1) There is no longer a bottleneck after the downpipe- on the wrx exhaust the downpipe is 2.5 inches then turns into 2.25 inches at the midpipe and stays that way all the way back. The STI exhaust however is 2.5 inches from the downpipe all the way back. Theory is that the turbo can breath a little better and is not getting forced so much

2) My Catalytic converter had a huge crack in it. Exhaust wasn't flowing cleanly through- This one I'm pretty sure has nothing to do with it but who knows.

3) I was told by the shop that put the exhaust on that my old Catalytic converter was an aftermarket one. So judging by the way that the previous owners took care of the car (lot of little neglect here and there) I'm guessing there was not a lot of money spent on upkeep. If the OEM CAT went I bet they did not replace it with a high quality one. So assuming I had a crappy aftermarket CAT on there I'm guessing that would inhibit good air flow.

So I'm probably wrong but would love an explanation as to what's going on. Hopefully I'm not just imagining things. Thanks for the lessons!

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