Stock catback is night and day quieter than CSS, CSS is on the loud end of the volume spectrum as it’s basically a OEM muffler delete (slightly less volume than a muffler delete).
CTS is Chinese/ebay garbage...just the most expensive of all the ebay/Chinese garbage available for these cars.
A good basic rule of thumb is whatever volume increase you saw with catback mods (regardless of what they are) the effects will be doubled when you add a downpipe. That’s why people end up running six different exhaust set ups because they over or undershoot their goals several times rather than spending the money once on what they actually want.
I can’t tell you how many hundreds of people ignore my recommendations and say “I’m just gonna run _________ until I get a ___________” and they end of with a stock pile of junk spending 2-3x more money in the long run.
For those new here I started out with a muffler delete way back in December 2015, then got a junk CTS downpipe for free I immediately cut up and fixed which lead to the development of the 17950 and eventually a wealth of knowledge pertaining to what works and what doesn’t on these cars. I’m not picking on people, I’m speaking from experience