On stock horsepower levels, the 2.5in tubing is fine. You are only making 300 crank HP. And the split to two 1 7/8th exit pipes does help already.

My silverado has 2.5in pipes off each manifold into a single 2.75in catback system and makes 330-340ish crank HP with a tune and thats enough flow.

The issue is the manifolds on the LS4, replace those and the downpipe and you are just fine.