changing downpipe

[avipolo16v]

i've already change my cat to magnaflow highflow cat and a supersprint exhaust

i want to replace the pipe between the manifold to the cat and the pipe between the cat to the exhaust to pipe with bigger diamter (from 1.75" to 2.5")

i know that a to big pipe will make the gas emmosion to slow

this is worth a try? someone here make this change?