[naz]

aka.. short answer.. it depends on your downpipe

long answer.. take a look at the pic below



there are little grey lines that show you what each aftermarket part replaces

do you see the gap between 'cat back' and turboback those 2 pieces (labeled downpipe and midpipe) are typically replaced by an aftermarket downpipe (some DPs like prodrive and bosal are 2 pieces, just like OEM.. but the idea is the same)

the complication comes in that flange that joins the piece labeled 'midpipe (the one with the 2nd car) and the one called 'rear exhaust pipe' (with the resonator)

the flange between the two is oval on the OE downpipe, and on prodrive DP and bosal DP and a few others.. but in some like helix (i think) the flange is flat... so obviously if you try to make up the helix with a flat flange with a catback like borla or prodrive or stock that is expecting an oval flange it will be a bad seal....

does that make sense?

so that's why the borla has that adapter piece.. so you can still use a flat flange downpipe like the helix with the borla. you might not need the adapter depending on the downpipe you purchase

and lastly.. if you see a downpipe that seems stupid cheap it could be because that's a 2 piece downpipe (just like stock) rather than a 1 piece downpipe (like most aftermarket)

personally, i like the 2 piece because it makes servicing the transmission easier (the 2nd cat pipe has to come off to take the tranny out.. such as during a clutch change)

i also like aftermarket DP that keep the OE heatshield flange so you can look stealth and hide that aftermarket DP from visual inspection when someone pops the hood

again.. my pick would be prodrive because it is wrapped, 2piece and has OE heatshield flanges

gosh.. you're so lucky. i'm laying it out for you in such detail. i had to search and read all of this on my own over 2 yrs :)