I have a full race 'downpipe' with a cat and plan on attempting to remove it without removing the cowl. I believe it's possible because it's been done on a ninth gen civic
DIY - O2 Sensor/cat removal without removing cowl | 9th Generation Honda Civic Forum
Having had a EJ205 Subaru WRX/2.5RS for 10 years I'm pretty used to a similar process on downpipes. I plan on using the old double box wrench trick on the bolts to the exhuast manifold/head ie using a 2nd wrench on the open end of the other one to generate torque easier. Because my box wrenches have a slight angle on the closed head it creates a nice angle to clear the cowl.

Soon as I get my 2014 ecu and get flashpro running in my 2015 I'm doing this and will post if I'm successful. Anyone running a 2014 and flashpro on a 2015 please see my other posts or PMs asking for instructions on how to do this for my dealer who seems less than knowledgeable.

 

Okay so I can confirm it's possible to remove the factor downpipe without removing the cowel; well at least if you have your air intake out like I did to swap in a Takeda. Here is a first draft of a DIY where I only teach the trick anyone who owns box wrenches can use to loosen the bolts on the top of the header without taking off the cowl. I assume that is why all the 9th gen civic guys do it? The rest of the bolts you can get to from underneath on jackstands. I will say I think the ones between the upper and lower cat and 2nd cat and cat back actually took me more time

1) Remove air intake
2) If you have big hands/arms like me (6'3" 220 lbs) you will need to first remove the sparkplug/coil pack cover to be able to fit your arm between the cowl and block

3) Remove upper O2 sensor with O2 sensor socket

4) Remove the upper heat shield from downpipe if you can't break the bolts with a ratchet then you will be doing your first attempt at the double box wrench 'breaker bar' technique that forms the basis for being able to do this

You will be using this to break 3 of 4 bolts free that hold the downpipe to the head/exhaust manifold
5) Remove passenger upper bolt

6) Remove driver side upper bolt

7) Remove drive side lower bolt

8) Remove the passenger side lower bolt using 1/2" socket and extensions pictured. I reach up under the wheel well with my right arm


Other tips:
You will need to remove the lower heat shield and support bracket from the 1rst/pre cat to let it fall out.
Just break all the bolts free then go back and loosen them