I recently cracked the Borla headers on my 05 STI, which has an open source dyno tune. I went back to Borla and found that the original slip fit UEL headers were no longer available for my car. A new UEL header, pn # 17217, replaced the old style cross pipe w/slip fit with a solid tube design, but with a corrugated section welded in on each tube on the drivers side. Most likely this change is to eliminate the stresses and tendency of the original design to crack at the collector.
I installed the new pipe and found that the car was down on power and sounded different. I questioned Borla and they said that there shouldn't be any change when replacing headers but could not directly address the effects of the design change on an engine tuned for the old style nor for the change in sound.
I have spoken with several tuners and their position is that any change in the tubes (length, diameter, or different sections within a tube) affect the tune of the specific header. They believe that the corrugated sections alter the flow/tune of the old UEL configuration changing the pulse frequency and thus altering the power delivery and sound.
The new headers have lost the UEL sound, which I'll miss, but when I retuned it the new design delivered a 3% hp gain, a 6% peak torque gain and actually expanded the torque curve 900 rpm with no other changes..... pretty impressive. I really like the power of the new headers but the exhaust sound is louder and less appealing.
One last comment, the new headers are not nearly as well made as the originals, substancially thinner (1/2 the weight) and the welds can best be described as splotchy. The tube attachment to the flanges is much less substancial than the originals and the flanges are less robust. I wonder if they're outsourcing these to China or elsewhere?