Most are recommending against it, because they seem to crack or loose seal at the head unless built really well.
Apparently a lot more mods would have to be done to appreciate the increased flow. Suppose if you're running some huge ass turbo you wanted to spin up...the increased exhaust flow might be more appreciated....to avoid lag. Not to mention a properly matched super Intercooler
The even runners are supposed to be good for increasing, but equalizing exhuast flow .. Guess its a matter of looking at how well the flange and runners are built, , the coating used, and more you probably already know.
But I believe MIke's correct, in that you'd probably notice more immediate results on a non-turbo. That's how it was for some of my cars(N/a straight 6, v8's) ....header was one of the first improvements on the engine for me.
Course you could always buy one to show us the dyno'd results...