The General observations

All downpipes highflow cats or not seem to increase the noise levels and turbo spool up is more apparent. With some models the noise is respectively much louder but all depends greatly on the entire exhaust setup.

Catless downpipes will cuase a cel light (cat inefficiency) which electronic fixes are availiable for 02-04 models (05' hasn't been confirmed to me yet probably still works fine)

Flatplate (stock):
Basically just a stock piece of mild steel plate cut to shape with a 2.5" hole for where the turbo exhuast is. Waste gate and expansion potentional is just not existant.

stock dp compared to bellmouth


Bellmouth:
Basically just one big oval shape that slow tapers down to the downpipes desired width. Its nice becuase it allows great flow for both the wastegate and the turbo exit.

looks like this


Split bellmouth:
Same as a bellmouth with a shaped piece of iron providing a wall between the two exhuast ports. Supposedly should give slightly better flow charistics but i don't see why its so much different than a regular bellmouth



Divorced wastegate:
A popular style providing seperate tubes for the wastegate and the turbo exhuast. I think this has great flow potentional personally maybe the best of all of them.