I'm guessing you have the exhaust necking down from 3" to 2.5" right after the collectors? Try this... run 3" pipe off the collectors at least 2 feet back THEN have it go down to 2.5".
Having the diameter change right off the collectors kills the mid-range scavenging effect of the secondary portion of the headers. From where the primaries meet at the front of the collector it needs to be consistent diameter at least a few feet back.
This is also the main reason why running open headers with no collector extensions loses power, same effect.