1959 black 270hp (9/2/69) 1981 Beige L81(10/20/80)
I never did any timed runs with the header sidepipes on my '67 427 but they sounded great and the engine was very responsive. Enter sidepipes in the "search" at the top of this page and you will get 25 threads relating to sidepipes. Header sidepipes have been thoroughly discussed here.
The answer to this question is going to be different on every application. For the most part, expect to loose a degree of low end torque if you run them with the least amount of baffle. Plus, they will be loud.
You should pick up a few ponies on the upper end and get a little more RPM out of it with everything being equal. I would not expect to have a seat of the pants difference in power vs. a good set of headers and free flowing aftermarket mufflers. They do look cool though, and that is worth as many hsp as you want when it is sitting still.
Hey, if this price is for new ones, may be you should order a dozen and get some discount.
You can't imagine the sound of a big block through Hooker side pipes. Just a deep grumpy loud bubbling and you can even hear the valves clicking . Myne is a solid lifter.
The way to go.
Do it. Gunther