Well, I guess it depends.
I think that if anyone is going to mod their car at all, it's pretty much a given that headers are a basic requirement in the overall picture. If you only add headers, their will be an improvement with them alone. But if you do anything else as well, the headers will prove to have a synergistic effect in that it will enhance those other mods as well. After all, breathing well is the goal of most nearly all mods you will do on an engine. And it does you little good to improve the intake if the exhaust then becomes the bottle neck.
All things considered, I have NEVER heard anyone say they wish that they had never put headers on their vehicle. And I can't think of any other mod you could do, performance wise, to your vehicle, that headers would produce a negative effect compared to the stock exhaust system.
Now that being said, if I were interested in just raw performance and was going to spend the bucks ONLY for one single mod to get the biggest horsepower gain in one fell swoop, I would have to say go for a supercharger like the Paxton or Procharger. You can find used one fairly cheaply, normally around $3,000 to $3,500. But on top of that you have the install and necessarily a tune afterwards. But all in all, this would be the one single mod, for the money, that would give you the biggest increase in power.
In my opinion. And there are drawbacks, of course. The rest of the drivetrain on a 350 horsepower car may not be up to snuff when you pump 550 horsepower on down the line. So things may break.............
