You are correct that there are some gains to be had, but once you have a decent set of un-restrictive headers any gain you get from there on is probably going to be a trade off.... You may switch headers to gain torque but will probably loose some HP in the process...or if you do some to gain some serious HP you will most likely loose some torque.

I sat in a dyno session with my bosses motor builder for 5 hours one day...it was the first 410 he had built after doing tons of really good ASCS motors....5 hours worth of doing a pull and changing a fuel setting, timing, or component swapping, and once we got the best curve and top end reading anything else we did always cost in one area to improve another.....especially headers