What he said ^

I think custom headers & exhaust systems are abit hit and miss in general.
If you're looking for decent power gains, custom isn't the best way to go.
Fabricating an exhaust that gives you power gains isnt easy.
There's quite a bit of research and development involved.
Requires alot of testing/dyno runs and tuning.

True, 'branded-off the shelf' exhausts will cost you significantly more.
But essentially you are paying for the R&D, testing and tuning that is involved.
I think thyre worth the extra money.
But at the end of the day its up to you.