the dreaded exhaust question (again)

[The Radium King]

hi there, i'm asking the dreaded exhaust question (again). i've seached the forum but can't find what i'm looking for.

i'm new to the boxster and new to this forum - hello everyone ...

i have a 2000 boxster s. i am looking to increase power; i intend to do the chip/intake/plenum mods and was thinking about exhaust. sound is secondary and exhaust components (typically) expensive.

questions:

1) it makes sense to me that a freer-flowing exhuast will reduce torque at lower rpm (less back-pressure) but increase hp at higher rpms when more air is moving through; is this valid?

2) the boxster s has a more tuned exhaust, and aftermarket exhaust will yield a smaller (if any) hp increase; valid?

3) where is most power increase found - header, sport cats, test pipe (secondary cat delete) or muffler? i could see headers being most beneficial, as more power would be seen through efficiency (equal length pipes)?

4) if i am increasing/improving airflow at the intake with improvements to the intake and plenum, would these benefits be increased with a freer-flowing exhaust (more are in, more air out)?

i quess i'm trying to drill down to whether exhaust mods are worthwile and, if so, which ones give the best bang for buck. and finally, whether a $76 ebay header is just as good as a $500 tuner header ...