Nick's Garage just dyno tested a 440 using factory HP exhaust manifolds, max wedge manifolds and headers.
Results:
HP manifolds---- 375 horsepower------ torque- 389
Max Wedge------ 410 HP ----------------- torque-446
Headers ---------- 419 HP ----------------- torque-463
The headers (1 7/8" tubes) increased 44 horsepower. Back in the day everybody claimed they only gave you 15 more horses.