Take a close look at the angle between the flanges and the primary tubes. The tube should follow the direction of the exhaust port which is not at 90 degrees to the tube. IAP headers were incorrect at one time. The headers with long primaries and shortish secondaries are (I think) more like GTA headers.
Claims of HP increase depend upon what system they are being compared with. The Alfa one piece manifolds from about 75 to 99 strangled the engine and anything decent is an improvement. The factory 2 piece manifolds are probably as good as any headers at low to mid rpm but you are unlikely to see them on a race motor.
I think that your choice should be based upon your engine preparation and beware of headers with large bore tubes which can kill the low to mid range torque.
85 GTV6 3L
76 Suzuki GT500
2011 Jaguar XKR
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