I don't know... I haven't tuned too many perrin header VF cars that had a torque peak near 4500RPM. I've also seen a lot of perrin headers leak from the slips or warped flanges, so I wouldn't be surprised at all if that's what caused the excessive lag.
The only real fair comparison I see here is the one done by EFI. It shows some healthy gains above 5k RPM with a bit of added lag compared to a factory header. Now I'd really like to see a fair comparison with a Tomei/GTSpec/MadDad header.