I am running 3" from the headers back, in my 426 Hemi. The car puts out 397hp at the rear wheels with open headers and 385hp at the rear wheels with the 3" exhaust hooked up. This was a 45hp improvement over the stock 2-1/4" exhaust. The engine is stock with factory original cam. The same exhaust probably would not be a good idea if I was running stock exhaust manifolds.
In the same vein, my Trans Am's 1972 455 HO (stock 8.3 to 1 compression) had 3-1/2" exhaust put on by some crazy person. It had stock exhaust manifolds and four adapters welded together to up the pipe size to fit the 3-1/2" pipe and get this: 4" inlet mufflers
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https://www.yenko.net/attachments/110429-adapter1.JPG
https://www.yenko.net/attachments/110432-adapter2.JPG
I think the guy must have found the mufflers and then figured he was 1/2 way to an exhaust system and had to use them. The car would barely run with that exhaust system. I will be going back to stock when we reassemble the car.