from all i have seen if you do not have headers either long or short tube then the gains from exhaust mods on the s/c engine are very small. not enough to affect the tune. im also wanting to do the exhaust mods but am stretched dollar wise so mine will wait.
now if your tossing headers into the mix i would get it done before the tune

someone correct me if im wrong but the big gains on exhaust mods on n/a engines are due to the scavenging effect on the engine. as the srt-6 is force fed the scavenging has minimal effect