Perrin 2.5 experience
 

I have a JDL catted eul headers, appropriate OFT tune and a Perrin 2.5 resonated exhaust. The setup worked, overall very well. Though the sound of the exhaust seemed inconsistent. I found that I needed to re-tighten the V bands every couple of weeks. So I reinstalled the stock exhaust. To my surprise, I found that the car had better throttle response below around 4k than with the Perrin though the Perrin breathed better above 4k or so. I also found that there was carbon between the flanges of the Perrin. On checking, I found that the flanges were not perfectly flat. I re-surfaced them and reinstalled the Perrin. The installation was easier as there was more surface area contact in the flanges and the the v-band clamps felt solid rather than vague at the same 6.25 lb. ft.. The sound was quieter on cold start, consistent and the low rpm throttle response was improved though I still think the stock exhaust is slightly better In this regard. Though the Perrin is a great exhaust, I seem to have preferred the stock setup and have returned to that....at least for now.

is perrin 2.5 catback? can you separate it between midpipe and mufflers just like you can with stock catback?

Lots of questions? There are lots of answers here but it takes some reading first to even know what questions to ask.
There are old threads that cover many newcomer questions because others before you have also asked the same questions. Read the stickies too. search is your friend even though this forum has a crappy search function

And go to the parts suppliers to see what is available.

some have pictures of the parts they are selling.
And down load the factory manual and look at pictures and drawings like the exhaust system .

In regards to your question, the order is: header w/ built in catalytic converter, overpipe, midpipe with 2nd cat n resonator, then the cat back section with a couple of pipes then the muffler.

As I understand it (someone correct me if I'm wrong)
Back pressure in the NA stock exhaust gives you more torque low down.
Larger exhaust in a NA car will give you less back pressure and less torque but a higher top end.

I doubt you'll find an aftermarket exhaust that is as good on the low end as stock because most are meant to focus on the ultimate hp they can get, which means sacrificing low end drivability. That is why companies like Ace have multiple header and exhaust options.

Surprised to hear that about the flanges not being perfectly flat. Did you purchase the exhaust new or used?

You must be really sensitive. I bet the difference in power is like 1-2 hp/ft-lbs at most. Catbacks hardly have an effect on power

I have noticed inconsistent sound on my 3 Inch Perrin Catback, though I haven't needed to re-adjust my vband clamps. It may depend on weather/temp of the car. I'm not 100% sure. Everything else is stock except the catback.

I did purchase the exhaust new. I just returned from an 800 mile trip and the stock exhaust was quieter than the road noise. On those odd occasions that the road noise was not so loud and I could hear the stock engine and exhaust it did sound a bit like agricultural machinery. So the Perrin is a more pleasant sound though louder and yes, the stock exhaust has more torque in lower rpms.

If you feel like it, perhaps at some point, remove the exhaust and look at the join of the flanges. Is there any carbon pattern. See if there is any carbon that makes it to the outside edges of the flanges. I was surprised on mine that some did.

Sorry sir, this is just not true.

When compared one right after the other, this is my experience. I don't know if the headers and tune effect this sense or not, but it is my experience. Perhaps it is just the more immediate throttle experience that gives this illusion. Now that I think of it, I had a similar experience with a Miata performance exhaust ten years ago or so. I tracked the car regularly and remember thinking that I noticed the loss of low rpm torque more than I noticed the increase in high rpm horsepower. Of course now I am thinking I need to do a before and after dyno and put the Perrin back on just to see how delusional I might be... and I am not ruling this out. That sounds like a lot of work though when I may not keep the Perrin anyway.

Dont also rule out the fact that because you believe there is more low end torque with the Stock exhaust that you in turn might drive it just a tad differently. Maybe you let off the clutch a bit faster or rev just that few hundred RPMs higher which gives the illusion that the stock exhaust has better performance.

I know for a fact i did this. I originally had the Nameless 3". I put the stock exhaust back on because i am going back to a 2.5" exhaust in prep for my Turbo Build. I noticed an immediate difference in driving.

After a little while i realized that all the extra torque and power was due to the fact that i was driving the car harder. Because noise wasnt an issue anymore i wasnt worrying that i would get pulled over for noise (i run catless header in CA so thats why i worry). Realistically i maybe saw a 1-2 hp difference and a 2-3 lb torque difference. but because of my subconscious driving habits it seemed like a HUGE difference.

I think this is a rational line of thinking. I was motivated to look again at the flanges and find that the flange of the muffler section still was still not really flat so I have been leveling it. The areas that have the braces are pulled down. Perhaps they try leveling the flange before they weld it to the muffler and it distorted. Perhaps next Friday I will have the time to do a before and after dyno. I really am curious what is really going on in there.Of course as we are talking about pretty small differences, I still won't know much for sure.