Didn't the TBR pipes have a crossover built into them? I have Cobras so I am not sure.
TBR built a 6X2 that had a crossover.
I believe he's asking about the TBR 6X6 which are similar to the Cobra 6X6 but actually claimed a slight hp increase.
Different pipe size may have had something to do with it. Maybe even the turnouts make a difference.
The bottom line is that TBR is a serious aftermarket tuned exhaust manufacturer. I get the impression Cobra was simply producing, among their many other products, a simple set of straight pipes.