I have a 05 gt with a vortech kit with dyno and tune. I added 1" 3/4 long tubes and full exhaust and it made less rwhp. I was told that it was because off the headers and that I should change the pulley is this true?

 

probably...... no backpressure won't hold any boost

 

the big dia. longtubes are for high rpm uses. more flow, so, sure jump up the boost and add a meth kit and you will see a big hp # jump.