I've been involved in the auto racing community for most my life so have some degree of familiarity with headers.. I have also installed headers on tow vehicles with mixed success. The things that seemed to hold true in all cases were:
1) it cost a lot of money to buy, install, debug (heat issues and leaks) headers
2) marginal gain in power
3) no long term gain in fuel economy
4) the heat destroyed the headers inside a few years (and I do not buy el-cheapo headers)
5) I generally ended up asking myself why I went through it.
If you are really hurting for power, I think that you will get some help with the headers, but the cost of entering the game properly will need to be paid..
Most rigs would benefit from dual exhaust with chambered mufflers and dual catalytic convertors (where required) more than they would from headers connected to a stock exhaust system.
My $.02 worth anyways..
Regards - Randy