Crzyhrse somewhat answered your question on the 2nd post of this thread.
straight through mufflers usually gives a tad more HP because there aren't any restrictions like the chambers of a chambered muffler, but chambered IMO sounds great in stangs... dont get me wrong, straight through like LM1's with an o/r X on 4v's absolutely sound AMAZING and not too raspy like on 2v's...
straight through are will always be louder and a lil more expensive (like a mid pipe with no cats- no restricitions) and chambered will something like an echo because the sound will resonate in the chambers. ive already posted my vids and i have a chambered stainless steel exhaust, if anything try searching youtube for LM1's, Borla Stinger's, etc