The T/A race headers are IMHO a better deal than the street replacements header, which is still quantum leaps above the stock Buick units.
Race headers don't have as much exhaust backflow into #6 cylinder (upsets fueling balance) and their longer primary tubes enhance low end torque, throttle response, and raise convertor stall. Not to mention that the V band clamps eliminate crossover leaks. All good things!
Boost raises the N/A horsepower by a factor of boost. If you make more HP/torque N/A, you will compound that difference with the boost added.
If you can make the same HP with less boost, the result is lower backpressure due to the fact that the exhaust backpressure is always higher than the boost, sometimes by two or three times!
Adding headers to a N/A engines only drops the exhaust pressure by a few lbs. but the HP gain is noticable!
Any turbo spools up to a lower boost quicker than a higher boost. So, if you make the same HP with lower boost, your transitory torque is also raised.
As wonderful as the T/A race headers are, they, as all other headers that I have seen, can be improved by porting and smoothing where the tubes come together and at all of the flanges/welds.
The gains vary with boost, production inaccuracies, and different lots of tubing.
My T/A race headers gained around 8% on my flowbench with a few hours of work.
Your results may vary, but it's my car, my time, and I think it's all worth it.
Back to that old adage: if you can't make 25 HP in one place, make 5 HP in 5 places.
Or Mr. Spool's favorite: "There's a lot of 11 second cars running around with 9 second parts."
A lot of that is tuning, and the rest of it is attention to detail. My favorite....
TIMINATOR