exhaust section has your answers if searched....ill give a brief explanation.
Cat back system vary....youtube is your friend for how it sounds.
Any DP with a proper tune will let you pass emmissions when plugged into the diagnostics as longs the codes are off and no CEL. If you have a strict visual garage you wont pass with any mods
Catback have minimal gains, stock is good for 300awhp. Headers not really worth it for a DD.