I just got off the phone with Brenspeed a few minutes ago and asked what is changed tune-wise when using longtube headers. Basically the ECU needs to be corrected for the change in the FRONT O2 sensor locations(having been moved back a few inches) so there is something tangible. A/F ratio/timing all those things stay the same, but there are calibration changes so I would get that checked by BAMA ASAP.
What calibrations is that? Something with the exhaust cams maybe?