I want to thank everyone for their replies.
I just received an email from Extrude Hone and this is what they said for performace gains on exhaust manifolds:
""Rick says that after processing the average airflow increase is 15% with an approximate 15 to 20 horsepower increase. The cost to Extrude Hone process your exhaust manifolds is $675.00 (per pair) plus shipping. The turn around time will be approximately two weeks""

I just wanted to pass on the information.