Funny, I just had a discussion about headers with the guy that does my heads because I was thinking my 408 might benefit from some larger tube headers. He builds 800+ HP NA small blocks (mostly Ford and Chevy) and he says that big tubes are a mismatch to smaller cubic inch engines. You need to keep velocity up in the system or you experience turbulence. He has spent a lot of dyno time over the last 30 yrs. and consistently finds that only high HP/large cube engines benefit from bigger headers. I also saw a recent dyno test on a 302 Ford on a TV show (maybe the one your referring too) and it showed that big tube headers did increase power above the torque peak by about 5 hp (not 15-20) but below torque peak it lost just as much, nevertheless they gave the impression that the extra 5 HP was a huge gain which it wasn't IMO. For a street or even a street/strip car torque is what gets it moving so if you give up torque for a few hp up top you gain nothing. JMHO