I will just leave this here for you viewing pleasure...
I learned the hard way when I installed the JBA shorty headers and got rid of the cats.... lost a good amount of torque down low. These are big heavy trucks and you want all the Torque you can get... HP sells cars.. but torque is what gets you moving.
I ended up having a new Y-pipe built out of 2.25" pipe which restored all my lost torque and then some. Factory Y-pipe on my 5.3l was about 2.5"... that 1/4" makes a big difference.
It's not about back pressure but rather keeping the hot gases moving faster to improve scavenging.
Also long tubes are better for higher RPM HP but not so good for lower RPM TQ which is where most of your normal driving takes place around town.