Grrrrrrr.... Just kidding
Well really to get a properly tuned header you need to know the VE, peak power, and peak torque. and to nail it down the timing events for the cam. Generally most headers just produce a reduction in pumping losses. But a tuned header could produce 40 to 60 ft/lbs of torque over a non tuned header (or more!). Thats not to shabby. If the header produced a 50 ft/lbs increase in torque on the above engine at 3000 rpm that would be worth 28 hp at 3000 rpm. Say you could do the same at 5000 rpm thats 48 hp (torque x rpm/5252). Granted a set of custom headers aren't exactly the best value per dollar in the world of horsepower but neither is a Victor JR intake on a bone stock engine. However between two identical engines except for exhaust the engine featuring a tuned exhaust system will have an advantage.
More specifically the above math (given correct inputs) could point somebody in the right direction when they are trying to select a header. Making HP is not just waving a wand over a hat and saying presto. It's really a matter of sitting down taking a look at what your trying to make and doing a little math to see what parts will work out for you. As another (and cheaper example) A Victor 5.0 intake has 11.5 inches of runner length. Which is tuned to take advantage of a 3rd pulse pressure wave at about 8000-8500 rpm. Add a 2-1/4" spacer in there and you've lowered the resonant tuning to 7000 rpm ( acutally you measure from the valve but it illustrates the point). How much is a spacer (especially if you make one yourself). Or this one for figuring out how much cylinder head flow is need to get a certain amount of hp (hp=.2575 x CFM x # of cylinders) thats at 28" of water, but could save you some bucks. Juggle things a bit and you could determine how much cylinder head flow you would need to make certain amount of power ( CFM=HP / 8 / .2575). For 500 hp you need 242 cfm of intake flow. Okay so now you know how much airflow is needed to make 500 hp. Armed with the knowledge you can select a cylinder head and save some money on porting. Or armed with the same knowledge and you need to have a set of heads ported, you have an idea of what needs to be done. That way you don't sell yourself a bunch of stuff that you don't need or get suckered into a bill of crap just because somebody says yeah I can get 400 cfm out of those heads and it'll make 500 hp.
True not everybody is building a top level competitive engine, but given some theory (and all this stuff is workable on a regular calc or the scientific calc shipped with windows)you can get a bit more out of your engine compared to the next guy.
Superchargers and Nitrous make for dumbed down HP (no flaming here) but they can still benefit (especially when the bottle runs dry
) from a healthy dose of math.