you are very wrong, stock cat back systems are VERY restrictive, tey definently gain the most power, followed by a O/R midpipe, which not only flows better than the stock H with 2 cats and 2 resonators, but it also weighs alot less. headers are the last on the list though. no to mention most people cant install them like most can a midpipe, or catback. and cost to install headers is about $500-900.
FYI, companies BS you on the hp gain that their catbacks give you. You're not gonna get more than 5hp from just an aftermarket catback over the stock... like stated above, ITS THE MIDPIPE that makes or breaks exhaust.
b/t an X, H, and Prochamber you'll most likely see gains upwards of 8+hp maybe even 12, depends on how the midpipe is constructed
X= more flow than H, but you dont gain much low end TQ, you'll all gain it back again in the high rpms, X's flow the best.
*Raspy in tone, Nascar like sound
H= Great flow, but not as good as x. Provides more backpressure than x, so you'll have more low end TQ, but you wont gain too much HP off it it.
*Deep, classic musclecar tone.
Prochamber= Best of both worlds, has both qualities of X and H
*sounds deep like a H