much better performance.... generally THE first thing you do to a motor to increase power, free up the exhaust... . this varies greatly from shorties to tri-y's to full length to tuned... pretty much a requirement if your gonna do performance mods to the motor, cam, heads, fuel, etc.. also primary tube size varies what performance you will get based on it's diameter..
a quality thickwall header and the right exhaust improves exhaust tone...
con's and potential con's......
you usually get what you pay for... don't be surprised if your $125 Summit headers get rustholes in a couple years... various ceramic coatings or stainless help alleviate that, and can keep the headers looking good for a long time... but can get pricey... headers can run $100 to $1000, depending quality, finish, etc...
also cheaper, thinwall headers tend to have a "tinny" sound...
also some headers, mostly long tubes can have fitment issues.... shackle clearance, auto shifter or clutch clearance, BB's have been known to need to framerail notched on occasion... thin flanged cheap headers can leak at the heads, blow gaskets out and leak down at the 3-bolt collector flange... also your starter can be subject to heatsoak... plugs can be difficult to service... in most cases, for single exhaust, factory y-pipes wont fit and will need to be modified...