I have a full Kooks long tube w/catted 3" Y pipe and 3" intermediate pipe. I want to replace my Corsa muffler with something much quieter. I've seen suggestions of a Borla XS series or possibly a Magniflow. From your experience what would flow best but keep the PoPo away? It would need to fit in the stock location too. Thanks.

 

First try putting a Magnaflow #14419 in the I-pipe, that'll quiet it down some.
If the Magnaflow bullet isn't enough then replace the Corsa muffler with a Magnaflow # ...?
Here's where you would have to get creative if you want to stay with a dual 3" outlet muffler since Magnaflow doesn't have a dual 3" outlet muffler with the factory outlet configuration but they do have #12590(5" x 11" x 22" case) which has dual outlets on the same end. If dual outlets isn't a must then a #125799(5" x 11" x 22" case) would be a option. IMO, these would be your best options as far as high flow/quiet options.
Also, if you want/need short a shorter muffler case length there's the #12249(5" x 8" x 18" case) or with a offset inlet#12259. You could go with a Borla muffler but their best options is the #40944 (4.5" x 7.875" x 16" case), and it's at least $30 more than any of the Magnaflow options.