Ok, now this is just a guess. I just saw a picture of the actual DC sports header for the Celica GTS. I belive I know exactly what the problem is just from looking at it!
It does NOT have the divider right before the flange. I belive that DC is having the same problem as AMSS did with the header because of this. Without the divider the exhaust experiences Turbolence (Negative exhaust current) as it is forced into the catback exhaust piping.
Sorry to say, but without the divider...that header is going to lose HP. If not that, VERY minimal gains and very sparatic.
I hope this is not the case...if it is I sure hope that they install a divider. By doing this, it has the potential to have very close to the same gains as the Trial Header!!!