Header tube size is HP dependent and/or for the task at hand. In other words, if you have a 550 HP build and your looking to maximize the power for the strip, it is a larger tube header than one for use on the street. Let it be known the only reason why a smaller tube header would be better for the street is a not for the need of maximum power but to help the over all feel of the car and engine to navigate the streets in a more relaxed manor. And this can very well be a border line thing.
On my small block, a stock 360 in a ‘79 Magnum, with bolt on parts, no cam swap, I could not feel the difference between the standard size 1-5/8 vs the 1-3/4.
Despite the know it all internet hero’s warning me of a big lack of torque and sluggish low end performance, it never appeared. This was later backed up by a header test in *I Think* it was MoPar Muscle and/or Hot Rod magazines where Steve Dulchich (again, IIRC....) did that very test which also included a set of TTI headers. While the dyno tests revealed there strength and weakness, the actual driving & driving feel was fine. The car lit up the rear hides without issue on command.
What are your build specs on/or estimated/actual HP numbers? On what size engine that is doing what (cruiser, drag, street stripper) in your car?