Dang Harry. My wagon motor makes 625 hp at 6500 rpm.
Will bigger headers make better power? Ya, likely. Though if you look at my 2" headers, they are mashed 1/2 flat on the bottom(thus the need for wheelie bars), and it made no ill effect to the way the car runs. Unless you set your chassis up to use all that power, then the fact you might loose some from not using 2" pipes will be a moot point.
My favorite saying: "Warren Johnson may see a difference..." Meaning, on a Pro stock dyno, you may see an increase in power if you split hairs really, really small, but beyond that will you see an actual ET improvement after the increase(and the money outlay).
Two short stories:
Friend built a set of headers for a '65 Fairlaine. Billy Glidden built the motor for the car and wanted the headers to dyno the motor. Billy gave my friend grief over a few tubes and their lengths and the loss of power(blah, blah, blah). Well, in this car there is only so much room for headers. Will the car really see that 5 hp difference in ET? Likely not. But Billy looks at things like that and wants the most out of any motor combination.
Another friend with 454 Windsor motor in a '75 F100. Spent the money to have 2" headers built with 3" pipes out the back. Was suppose to be all that for the 600 hp he makes. Truck did not pick up ANYTHING in ET over the generic 1 7/8" headers. Lots of money spent, for no gain.
Just throwing this out there for food for thought from my experiences. I get the want for Stainless so you don't have rust issues. Size wise? Don't sweat that as much.
My .02
