would it be better to straight pipe my duramax or get an exhaust system? how much does back pressure effect turbo diesels?

You don't need any back pressure since it is turbo charged. I've had a straight pipe on mine since 500 miles. Plus on my other 3 cummins I had.

would it be better to straight pipe my duramax or get an exhaust system? how much does back pressure effect turbo diesels?
if youre gonna be towing or plan on building the tranny and runnin big programs i would go with a full 4" exhaust to keep egt's down. but if you just plan on playin around with a predator, edge, etc i wouldnt spend the extra money...just cut the muffler off