Don't get me wrong a plog, dp, rear ported mani setup can give you some decent gains.

For example:
My previous GTP put down 271hp/325tq with:
1.9 moded rockers
plog/ rear PEM, 3" dp with high flow cat
180 tsat
ZZP 1.0 PCM
3.4 pulley
SSIC

Just makes me wonder what I could have put down with headers.